Choosing an Amp
The main things to consider when choosing an amp are price, your core tone desired, versatility, reliability, and power. Most amps are designed for several tones and have a tonal signature that is difficult to find elsewhere. This doesn't mean you should or shouldn't like any amplifier's signature characteristic - you may prefer other aspects of it, or a channel or feature that doesn't get the same love as other tones it can produce. However, when trying to figure out what amp you want, it's a lot easier to listen to lots of clips and choose which tones you like and narrow that down to a list of amps that can do that tone well than it is to aimlessly try every amp on every channel.
The modern amplifier generally has 2 to 4 channels that you can switch between. This will give you that many distinct tones that you can access on-the-fly. Some amps have completely independent controls for each channel, while some amps share some controls across channels which may make them less versatile. Beyond that, some amps have controls that affect all the channels. Outside of tweaking the controls provided on the amp itself, using boost/overdrive pedals, EQ, or other effects can be used to strongly alter the tone.
Contents
Budget
Quality guitar gear is very expensive, and it always feels like you never have enough, or the right gear. An amp is typically a major purchase and the centerpiece of your guitar tone. I'd always argue to spend more on your amp than anything other than possibly your guitar(s).
If you are a tone junkie, you may want to consider a good modeler. For the price of 1-2 quality amps, you get several dozen to hundreds of models.
Remember that your amp purchase isn't your only expense. Tube amps have to be retubed and possibly maintained every so often. If you need additional effects, such as boost pedals, noise gates, or delay/reverb, those can add up quickly. So can cables, pedalboards, gig bags, cases, etc. Write it all out, then set your budget. If you are a professional and need absolute reliability with bigger gigs, you should also have a backup amp. If tone is crucial or budget isn't a big concern, you can fully duplicate your rig; but practicality often necessitates a watered down, "get-through-the-gig" backup rig.
A major part of amp tone is what cabinet it is paired with. Cabs can range from fairly cheap to quite expensive. If it's a key part of your amp's tone, you may have to spend a bit of money here - the expensive amp but cheaper cab solution may not be a solution at all. Or you can consider looking smaller, such as getting a 2x12 vs. a 4x12. I use a modeler with a FRFR wedge monitor, and it was quite cheaper than a real guitar cab.
Pre-Amp vs. Power Amp
An amplifier generally consists of two main pieces: the pre-amp and power amp. The pre-amp is generally where the tone-shaping takes place. Modern amps also feature almost all of their distortion here, where it has more control. It also allows you to separate the amount of distortion you can dial in from the amount of volume. Older amps relied on power amp distortion, which meant you had to push the amp to full volume to start getting distortion.
That said, the power section isn't completely transparent, and its tone will vary from amp to amp depending on the design, even when it isn't pushed into distortion.
Overview of Common Channels
Clean
Clean channels are designed to get a relatively clean (undistorted) tone. That being said, some amps' clean channel is still a bit dirty. Sometimes this channel will feature a gain control to dial in pristine cleans vs. mild crunch, but usually not. Even if there is no dedicated gain control, it may still be possible to push the channel into distortion by maxing out the available EQ and channel volume controls.
Simply because tones are clean doesn't mean they'll sound identical across amps. Some amps will have a more compressed sound than others, or have more sag or otherwise alter the attack of the inbound guitar signal. This can get very complex, with the compression affecting some frequencies more than others. Some amps are known to be "warm" while others "sharp" or "cold".
Its also important to pay attention to the tone of the notes as they sustain. Some amps are said to be "organic", while others are "sterile".
Crunch
Crunch is basically a mild to strong distortion, but it usually responds more to attack and playing dynamics than a lead distortion. This channel usually has a dedicated gain control. At lower levels, it will sound like a dirty clean channel. At higher levels, it has a fairly saturated distortion; however, the breakup tends to be a bit noisier or harsher than a lead channel's. In other words, it is less compressed and dirtier than the lead channel, but more compressed and distorted than the clean channel. The distortion also tends to sound more nasal and vintage than modern and tight.
Lead
The lead channel offers you the maximum amount of gain and the most modern tone. The response is tight and the distortion can get very saturated while remaining fairly smooth and harmonic. While it's typically labeled Lead, this channel works well for both leads and distorted rhythm where heavy distortion is preferred over prominent attack. It will almost always have a dedicated gain control to dial in the desired amount of distortion and sometimes has additional tone-shaping controls that other channels don't.
Lead 2
An additional lead channel may simply offer more versatility by being a complete duplicate of the other lead channel, allowing two different sets of settings (Gain, BMTP) to be instantly accessible and tweakable. Sometimes the additional lead channel will have a different signature to the tone, including different dynamics, breakup, aggression, tightness, and clarity.
Modeling vs. Analog
Traditionally, amps have always been analog electrical devices, focusing both on signature tone-shaping (including adding distortion) as well powering a speaker cabinet. This meant that the amount of tonal control was limited by the complexity of the circuit - the more channels and options, the more expensive the amp would be. Early amps had 1 channel, limited EQ controls, and little or nothing else.
As the cost of electronic components became cheaper and more powerful and computer science developed sound research and methodologies for signal analysis, digital processors began to offer not simply effects, but algorithms that emulated the tone-shaping aspects of amplifiers and mic'ed cabs/speakers. This allowed players to avoid the power aspect of amplifiers altogether, sending their output into DAWs for recording or PAs and monitors for live sound.
Unlike analog amplifiers, modelers could change their tonal processing simply by loading different algorithms, using the same circuit. This allowed them great versatility. Lots of algorithms were made, many designed to emulate specific analog amp models. They could also have their speaker/cab/mic emulation turned off and be connected to the power sections of traditional amplifiers running into traditional guitar speaker cabinets.
The downside of modelers has always been the quality of the algorithms - early units could clearly be differentiated from the analog amps they attempted to emulate, often sounding "fake", failing to replicate certain aspects of analog amps, or displaying artifacts from the digital algorithms such as aliasing or "fizzy" (noisy) frequency ranges. Current generations for the most reputable brands, however, are very difficult to differentiate from their analog counterparts.
Other downsides for modelers are increased complexity and occasional needs for an additional power amplifier. On the other hand, they don't require maintenance or tube replacement like tube amplifiers.
Tube vs. Solid State
Early amplifiers used vacuum tubes exclusively for all their amplification, including in the pre-amp section and the power amp. Later on, transistors (aka solid-state) began to replace tubes for many applications, due to smaller size, cheaper costs, and no necessary replacement or maintenance. However, their application for audio amplification operates somewhat differently than tubes. Many of the things one might find undesirable about tubes from a technical viewpoint are actually desirable to many musicians.
This is particularly true when pushing tubes past their maximum output range, which results in clipping, or distortion. Tubes' distortion is often characterized as "natural", "musical", "organic", and "warm". All that really means is that it sounds pleasing to the ear. Transistors tend to be less pleasing, with a harsher distortion. Tubes also offer aspects, like sag, that cause the technical replication of the amplified signal to differentiate from the source signal over the life of a note. Again, many find this as a positive benefit for musical purposes.
Various technologies have been developed by amplifier manufacturers to get the tube tone out of solid state technology; however, none of them gained enough traction to be considered at the same quality level as tube amps. To this day (mid-2014), the most popular amplifiers rely on tubes for both tone and power. That said, they are also more expensive, require tube replacement and occasional maintenance.
Solid State is now quite popular for transparent power amps with plenty of clean headroom to amplify a modeler which is already modeling the tonal impact of a tube power amp. This can be run from a modeler with no cab/mic simulation into a guitar speaker cabinet, or with cab/mic simulation into a passive FRFR monitor. Active FRFR monitors nearly always have their own such solid-state amplifier built into them.
Watts vs. Volume
Amps measure their power in watts, typically 25, 50, 100, 150, or 300 watts. Some new, smaller "lunchbox" amps only have 2, 5, or 10 watts of power. These wide discrepancies may seem like they translate into similar differences in potential volume. However, this is deceiving.
First of all, identical claimed wattage doesn't mean equal volume. Power can be measured by peak power, or by RMS (root mean square), or some other way to define it. RMS is a statistical measure that gives a more accurate picture of what an amp can actually sustain, while peak power only lasts for an instant, which isn't even perceivable. A 50 W and a 100 W amp measured by peak wattage may peak at their respective ratings, but both might have an RMS rating of only 10 watts.
Second, even using RMS power, different designs in amplifiers can exhibit different perceived volume. Tube amps tend to distort differently than solid state amps, which equates to a 100 W RMS tube amp sounding much louder than a 100 W RMS solid state amp.
Third, power does not translate linearly into volume. Double the wattage may only mean 25% more volume, as measured in decibels (+3 relative db). A 20 W amp may sound only half as loud as a 100 W amp. Now, that 25% can make a huge difference in a band context, where you would easily be drowned out. So it does matter, but the numbers don't tell you the complete picture.
This article explains everything pretty well.
A Brief History of Distortion
Early Experimental Distortion Techniques
Early amps weren't designed to produce thick, saturated distortion. Many artists discovered the phenomenon by accident, using amplifiers that were damaged in transport. Some purposefully altered their amplifiers by doing things such as loosening tubes or poking holes in their speakers. Others cranked up their amps as high as they would go, producing a mild overdrive. As the peaks of waveform exceed the amp's maximum output, the peaks would be compressed, or clipped, creating a more square-wave type waveform, which is the equivalent of adding lots of harmonic overtones.
Overdrive
Once artists and manufacturers began to appreciate distorted tones, they found various ways to create them (without irreversibly damaging an amp or speakers). The simplest was to push an amplifier beyond its designed maximum output, creating an overdrive distortion, by running the amplifier at full volume. This could be pushed further by using a high-output humbucking pickup. These were the earliest ways of achieving overdrive distortion. It would later be revisited in the 70's by using solid-state distortion pedals to boost the signal in front the amp, pushing amps much further into overdrive. Amp manufacturers also realized the potential desirability of overdrive distortion, designing their amps to overdrive more easily and with a smoother, more consistent, less noisy distortion.
Fuzz/Dirt/Distortion Pedals
Overdrive was originally reserved for a mild distortion, but artists wanted more. Enter early solid-state distortion pedals: fuzzboxes. These things produced a harsh, noisy, "dirty" distortion but offered artists a much greater degree of distortion. Solid state pedals would evolve more and more, getting away from fuzz and offering more tube-like distortion, in the form of overdrive and distortion pedals.
Recently these vintage devices have come back in style. When choosing an amp, one must take into account how it will pair with such devices, if that's the sound you are looking for. There are no hard rules - a healthy dose of research and experimentation is in order.
High-Gain Amps
In the early 80's, amps started to incorporate heavy tube overdrive in the pre-amp section using the smaller pre-amp tubes, with variable gain/drive controls. This allowed real tube overdrive, without having to crank volume to ear-bleeding levels. These started the modern trend of high-gain amps. Various refinements were made, such as cascading gain stages across multiple tubes, offering heavy, saturated distortion without compromising control and minimizing noise and other unwanted warts.
Many artists still run boost pedals in front of high-gain amps, not from any necessity for more distortion than the amp can offer alone, but more due to its impact on frequency response or other tonal/response changes that enhance the amp's distortion. Most amps have distortion voiced for a middle-of-the-road rock tone, while metal artists prefer a tighter, brighter tone that doesn't get muddy on palm mutes. Often using a boost is the easiest way to get there.
Modelers
Most recently, companies are developing digital devices using algorithms to emulate various pedals and amps (including pre-amp and power amp tube distortion). This allowed artists access to a greater range of combinations of pedals and amps and experimentation that would be impossible in real life - things like running amps at full volume with the tubes in class A operation biased as hot as possible or daisy chaining multiple amps.
Effects Loops
Most modern amps feature the availability of distortion in the pre-amp. Additionally, even when using distortion pedals in front the amp, the pre-amp still provides compression. Many effects should be processed completely after any distortion or compression affects the signal. For example, the delay trails of a delay effect should gradually decrease in volume until they are inaudible. With compression, the trails would have a more consistent volume level until suddenly dropping off. Similarly, reverb would seem to stagnate rather than decay. Distortion would be even worse, with delay trails and reverb being distorted just like the original signal, creating a noisy mess.
An Effects Loop gives you the option to run effects between the pre-amp and power amp. In some cases, the power amp may still be providing compression or distortion; but it's usually less than the pre-amp.
Amps traditionally have serial effects loops - the signal leaves the pre-amp from the effects loop send, is processed by any effects in the loop, and returns through the effects loop return to enter the power section. Most effects have mix controls - they split the signal in two as soon as it enters the effect and only process one of them. The resulting "wet" signal is mixed with the unprocessed "dry" signal, with the proportions determined by the mix control. 100% is totally wet, 0% is totally dry, and 50% is equal mixes of both. If all your effects have mix controls, or you always want to run your effects 100% wet, then a serial loop is fine.
A parallel effects loop features a mix control, using the same logic as individual effects - you can determine the mix of dry, where the pre-amp signal goes straight from pre-amp to power amp, and wet, the signal processed by all your effects in the loop. 100% mix is identical to a serial effects loop, while 0% is like using no effects loop at all. The big benefit of a parallel loop is when you use effects without mix controls, which was typically the case with vintage effects.
What you have to watch out for with a parallel loop is when effects introduce lag into the signal, such as with some digital effects that, by nature, buffer the signal time-wise. Even a small lag in the signal will create a comb filter with the dry signal, where regular nulls occur throughout the frequency spectrum, particularly in the highs. The tone will sound dull or not as full as it should. Also, effects like tremolo or others that alter volume may sound quite awkward when blended with a dry signal.
Signature Tonal Aspects
Fender
Fender is one of the first big names in amps, producing many classics still sought after today. Their vintage tone is typically defined by shimmering cleans with a nice fat bass response. Pushing them into heavy overdrive can be a rather unpleasant experience for many models, but with the right pickups, style, and/or boost in front, they can deliver excellent lead and mild crunch tones. Keith Richards and Stevie Ray Vaughn are some notable users.
Marshall
From the early 70's to today, Marshall basically dominated the amp market, by delivering the crunchy, "British" tone. The clean tones were sweet, but what set the amps apart was they could be pushed into overdrive more easily and without getting nasty. Think Clapton, Hendrix (when he's not using a fuzz pedal), Ritchie Blackmore, and Pete Townshend. Their sound is somewhat nasal but delivers a relatively aggressive distortion, especially when boosted. This is how early Eddie Van Halen, Randy Rhoads, Zack Wylde, and pretty much every 80's hair metal band got their sound. Other notable artists include Joe Satriani and early Steve Vai. Many modern small-shop amp manufacturers got their start by modifying Marshalls to have greater and more focused gain.
Vox
Vox is popular for their AC-15 and AC-30 amps, which are similar to Fenders for their pristine cleans, but also deliver very controlled-sounding distortion. You get an amazing range of distortion simply by using the guitar volume knob, and the distortion sounds great on both single-coil and humbucking pickups. Ultimately, their sound is defined as vintage, but I feel they can be dialed in for somewhat modern tone.
Orange
Orange is another British company who delivered the "British" sound - somewhat vintage but big distortion tone available. What separates them most from the Marshall tone is that they are somewhat darker, with a bit more of a fuzzy sound to the distortion.
Mesa/Boogie
Mesa/Boogie pioneered the "American" sound, shifting away from the crunchy, nasal British sound to something more punchy and crisp (and sometimes "scooped" mids). Their Dual Rectifier line dominated the late 90's and 2000's, defining much of the alternative and (even moreso) the alternative metal sound. They delivered tight, thick bass and searing distortion with a snarling presence. The Dual Rectifier had a somewhat buzzy distortion, but this could be turned into a searing crisp distortion with a boost. Their Mark series was known for sweet leads and tight response - it was used by both Carlos Santana and Metallica. John Petrucci of Dream Theater is a notable endorsed artist, as well as Korn, Lamb of God, and Foo Fighters.
Diezel
Diezel became a big name in the late 90's, popular in the metal community for its huge distortion tone. They are somewhat dark, but quite crisp and saturated. Ultimately, the amps are very versatile and shouldn't be labeled by a specific tone. For me, the main thing I notice is that the distortion seems big - the entire frequency range is represented well and just sounds monstrous, but the response is very tight.
Bogner
Bogner makes some expensive amps, but most fans will tell you they are worth every penny. Their main amps, the Ecstacy and Uberschall, cover fairly different grounds, one being more sweet and the other evil; and each amp has a lot of versatility, with many different channels and unique ways to tweak them. If I had to pick one defining characteristic it would be quality. Every channel, every tone you can get out of these amps is top-of-the-line for that respective field. The Ecstasy is particularly known for its fat cleans that rival Fender, its sweet crunch and leads from its blue channel, and its searing distortion from the red channel. The Uberschall's distortion tone is quite unique, combining a thick, rumbly low-end with a buzzy distortion yet retaining a sweet midrange. Notable artists include Steve Lukather.
ENGL
The other big name in German amps, ENGL rose to success with its buzzsaw-esque distortion. It produces a very thick, big distortion, like Diezel, with a signature buzz and high-frequency rasp that sounds brutal but avoids being overly harsh. They have a wide range of prominent amps - some are darker with thicker low-end, some sound more crunchy vs. saturated/compressed; but overly the tight response and unique distortion tone make them stand out.
Peavey
Peavey's main success came in the early 80's by designing Eddie Van Halen's signature amp: the 5150. Still a favorite in the hard rock and metal community today, it features a world-class distortion tone that fills the entire frequency spectrum and gets exceptionally saturated without getting noisy or suffering from a loose response. Many metal artists use these with a boost in front to get an extremely tight and thick distortion. And they are relatively cheap (in America, at least). The 5150 II added a couple extra controls and improved the clean channel. When the 5150 naming rights reverted back to EVH, Peavey renamed this line the 6505; 6505 = 5150 and 6505+ = 5150 II. To confuse the matter more, Van Halen's own company has made the 5150 III amp, which is a bit different with 3 channels and different response/voicing.