This is a problem inherent with both my Spider IV HD 150 and my Spider IV 150 combo. The master volume control is uber-sensitive around the 8-9-10 o clock position. The volume output rises and maxes out very quickly. Also, the pots move so freely, that at high volume levels, the master volume knob can and will move from vibration.
The only solution I have come up with is to try to set the channel volumes of your presets lower, and I put a #10 o-ring behind the knob itself, between the knob and the face plate of the amp, which offers a little more physical resistance (depending on how hard you push the knob back on the shaft of the pot after installing the o-ring) and my master volume no longer moves when I don't want it to. It also makes the master volume a bit more manageable.