Probably V1 for cleans (the tube closest to the input jack) and/or V2 for dirties (the one next to V2). 12AX7s are actually TWO amplification devices in one, so each tube usually performs dual tasks. Replacing a 12AX7 with a 12AT7 reduces gain by 40%, while a 5751 reduces gain by 30%, and a 12AY7 by 55%. Try Googling "Mesa Boogie schematics".
