I turn the master volumes on my M4s all the way up, and just use the master volume on the power amp (MOD50) as the master volume. So the individual gain and master controls for each module and the master volume on the power amp are the only volume controls that I ever adjust. But I can't turn the master volume on the power amp past about 1 o'clock without overpowering feedback.
http://www.geocities.com/tonesponge/EgnaterRig.html?1170011462115
Part of the reason I set the master volumes of the M4s all the way open is because I need the ouput from those modular preamp rack units to match up in level with the output from the MOD50 preamp. If I turned the master volumes down on the M4s, then I'd have to crank the master volumes on the individual modules to compensate, and I have them all the way open already on most of the clean channels. So I really don't have much choice.