setting the master too high could drive the fx loop too hard, getting some unwanted, muddy sounds. also, when you're using the output control, the channel masters become, more or less, a second gain control. setting the MV too high can muddy up your sound for this reason too, just like setting the gain knob too high would. but, setting it too low might not not give you the "balls" you want in your tone. IMO, you should set the gain knob to taste, the channel master to about 10:30 - 11:30, and use the output knob to control the actual loudness of the sound.