Sound imbalance w/ integrated amp

Okay, I recently bought a used ADS integrated amp. It arrived poorly packaged and it's back left fin got whomped (sorry for the technical term). I plugged it in and started testing. After about a half hour of switching things around to make sure I nailed down what was the problem I hit on a sound imbalance inside in the integrated amp. The seller gave me my money back, which was nice, and told me to keep it or chuck it. I figure It would be a fun project to try to fix it and, anyways, what do I have to lose? Problem is that I need a little advice as to what I should be looking to repair.

So, my question is this: what would cause one channel to sound much
louder (or weaker) than another? I KNOW this: it isn't the speakers, I
tried different ones. I switched the speakers, I tried different inputs, and used both the A and B speaker outputs. Always the same problem with the same channel. So what would cuse this? Could this be a sign of decaying capacitors? I can't think of anything else. The balance knob works; if I turn it All the way to one side I can get an almost balanced
signal. So the balance works, in theory. The inputs work. The speaker outputs work.

Any suggestions? I will gladly supply other info. If I have missed anything I will gladly elaborate.

Thanks in advance for any help,
Aaron