Given everything we know so far, I suspect the left channel relay is closed. Personally, I would just replace it...but you can try to open it up (pop off the plastic cover) and see if you can work a strip of very, very fine sandpaper (1200 grit or higher) between the contact points. Or, just try a strip of notecard or typing paper soaked in deoxit. The coating on these points is VERY thin, so only one or two passes.
If a contact point is indeed stuck in the closed position, replacing the relay is a much better idea, but cleaning the points may work (assuming they are not fused from your earlier misadventures).
Here is a thread about getting the cover off:
http://www.audiokarma.org/forums/index.php?threads/relay-cover-shucker.19413/
If you replace the relay, I would recommend replacing all the electrolytics on that board while you are at it. I seem to remember that one or two of them are bipolar, but they are not indicated as such on the parts list in the service manual. So check the values you need by looking at the actual caps on the board before ordering.