Hello, I would really like to control the amp gain with a expression pedal.
This is a feature that was available on some mesa boogie mark II amps see this link
William G. Sacks has written, "A cool and little-known option with the Mark IIB (or original Mark II): while the effects loop isn't worth much as such, you can run the output of a volume pedal (without your guitar being plugged into its input) into the 'return' jack, and get on-the-fly preamp gain control! This is a feature that very few tube amps have ever had, even inadvertently - it uses the pot in the volume pedal as a passive limiting device, such that when the pedal is 'wide open,' it allows the preamp tubes' full voltage to pass into the power section. As the pedal is backed off, it decreases the amount of preamp signal allowed through the 'loop,' cleaning up the sound. One of my best friends sets his clean channel's preamp to 9 and uses an Ernie Ball pedal to vary the overall tone. Oh, I wish we could convince Mesa to make this a regular option on their new models..."
While I didn't own this amp I've used this feature on a digitech RP 500 where you can assign almost EVERY parameter to the exp pedal. The result is incredible since you can have so much variety in a single patch, great for blues and jimi songs.
The added bonus is that you can let ring a single note and press your exp pedal down while it fades out for ultimate sustain , feedback & great larsen. Not to mention that you can switch from rythm to lead as you do with your volume pot but with great clarity without adding mudiness. To compensate for the changes in volume, a 2nd exp pedal could be used.
going further, it would be great to be able to control more parameters on the kpa with the exp. pedal.
Hope you'll enjoy this great feature.