My Spider II 15w has a problem: When the amp is turned on, AND any time a new channel (clean, crunch, metal, insane) is selected, the Channel Volume seems to go automatically to 100%, producing instant, overpowering feedback no matter what. As soon as the CHAN VOL knob is moved, it goes to the indicated volume (so I can have my hand on the CHAN VOL knob when I switch channels and immediately turn the knob to kill the feedback, but still for a split second the amp puts out as much noise as it's capable of). This happens regardless of what guitar is plugged in, and even without a guitar plugged in. Also, the MASTER knob has no effect (even at 0/10, this problem still happens). Any idea if I can fix this?