The Gain Plug-In
Setting constant volume levels in Mainstage can often be unreliable. Either the fader doesn't amplify sounds loud enough, a fader jumps way out of position, or the fader gets moved and doesn't revert back to it's original value.
A couple years ago, I was looking for a solution to this problem and I eventually settled on using the Gain plug in. This plug in is a stock insert and can found under the utilities section of Apple's plug ins. Gain also has a bunch of other helpful audio features including a left and right phase inverter, a stereo channel swap, and a sum to mono feature, in addition to the gain knob.
The beauty of this plug in is that it's hard to move out of position. The only way to change levels is to actively go inside of the plug in and move the knob yourself. No chance of someone accidentally moving it and blowing up their levels.
My preferred method of using this plug in, is to set all faders to 100%, except for the master fader. Then when I start balancing, I instance the plug in and get to work. This plug in uses virtually no DSP, so you don't have worry about overloading a session. Now you can sleep well at night, knowing you're not resting the entire show on something as finicky as the channel faders.