Mark I/II:XL:XL+:Not a big change from 2.04. The power amp modeling is slightly improved. The biggest change is the operation of the Dynamic Presence control. Transformers interact with the speaker. When the transformer is driven hard this interaction changes. It's dependent upon the transformer, the speaker and even the cable to some degree. The net effect is that the high frequencies are boosted but it's a dynamic boost.Turn up the Dynamic Presence knob and you'll hear the effect of the interaction. Amps with small transformers connected to certain speakers tend to really do it but, like I said, it depends on the speaker so I made it adjustable. For example, our AC-20 Deluxe into a Celestion G12T-75 exhibits significant interaction but into a Jensen C12 does it much less. Our 50W Plexis do it a bit but our 100W Plexi doesn't. Amps with "big iron" don't really do it much. Amps with small transformers that are easily overdriven do it. So... use your ears. As you turn it up you'll hear the transformer "grind". It's a sort of scratchy, gritty but cool sound.