this is probably stupid, but whats the difference between volume and master volume on my amp?
volume = before the preamp, think of it as a "gain" or "drive" control

master volume = after the preamp

using the 2 togethers allow you to distort your sound in the preamp but still keep it relatively quiet
Volume effects the tone, how strong your gain and effects sound.

Master volume changes how loud you are without changing your tone.

They do this so you can change the amount of distortion without making your ears bleed.
well.... one volume knob is the master and the other is its servant bringing it whatever it likes like the distortion you need to really put some over drive on that riff your gonna play after you read this..... right? and the master in return will make it louder there fore making the servant happy right before it is executed in the dungeon of distortion and harmonic. in short listen to what seljer said and youll be ok
AFAIK, the volume knob determines pre-amp volume and the master volume is the power amp volume.