I set the red channel volume (modern high gain) to 10:00 and all the other volumes to balance that.
Turning the channel masters up too high overdrives the FX loop, and keeping the loudest channel moderately low helps preserve clarity and dynamics.
Further, Mesa's tend to sound better if you can get the master output above about 10:30, which is made easier if you don't have the channel masters set extremely high.
If you bypass the FX loop then everything I said above is moot as it bypasses both the FX loop and the master output circuits.