Do you have a voltmeter? Can you check the rails (the positive and negative voltage, assuming the amp is DC coupled, that is)? If it's only one one channel, this probably isn't the problem, though. If you can figure out which is left and right circuity, you can play a test tone through both and go from the input to the output to see what is different. If you get a radically different voltage somewhere, that's your culprit. Typical amps are three stage -- transimpedance (voltage to current) (typically, using a current mirror), transconductance (I think -- I need to look this up; current to voltage), then a current amplification stage.
