Well I attempted to localize the trouble to a particular board, I injected test signal at the aux input and measured the output of each channel of the control board (measured at the inputs to the power amp) and the left channel signal looked a bit strange but the wave height was the same as the right while I moved the volume control. I then injected directly into the inputs to the power amp board and the sound volume was the same on both channels.(Leading me nowhere) During the test for the control board I had to move the volume control up and down and when I do that sometimes the problem is not noticeable anymore. It comes back every time I shut the unit off, so I need to inject directly to the power board when the problem is evident and without moving the volume control much. Also, the power board is installed properly and there is not much room to probe with the board installed that way. I have not yet devised a good way to support everything safely with the power amp board open, the heatsinc and finals are heavy and those leads are easily shorted, etc. Any ideas would be appreciated, if anyone has worked on this particular amp before. Thanks. By the way, I have decided to upgrade the opamps in the tone control circuit anyway. I found good converter boards and ordered the usual replacement - the OPA604AP. I've also replaced Qi1 and Qi2 with no changes to this problem. The overall sound of this amp seems to be deteriorating as I replace components, which is disappointing. Looks like I will need to continue to recap and replace components as I learn more about what is causing this problem with the left channel.