In the Mark amps, the mid knob doesn't really adjust the mids like you're used to, it adjusts how the mid sits in the mix, and when the gain goes up the effect goes down. Try turning the knob again when you're in the clean channel. If you still hear no difference at all then it could be broken. If so, I'd just go ahead & send it off to Mesa to have Mike B. go through it. Worth every penny.
