Changes

Closed back cabinets are particularly susceptible to resonance. This is not a bad thing...depending on the size, shape, and material of the cabinet. Resonance will occur at several places in the frequency spectrum, but mainly at a certain low to midrange frequency. The resonance doesn't simply emphasize a certain frequency but affects the way the speaker itself behaves, changing the character of the speaker's tone. It cannot be emulated simply using EQ/filters - it is a non-linear response, affecting tone across time, not simply transforming the current state. I can't tell you much about exactly how it all works or what to expect. I would recommend trying to find a tone comparison between Marshall 1960 and Mesa/Boogie oversized Rectifier cabinets, using the same speakers and mic'ed the same way. Some modelers allow you to tweak the resonance of emulated cabs virtually, effectively allowing you to tweak the cab's construction in real-time. This is great for dialing in your dream tone, although they tend to sound weird when fake when you deviate too far from center.