Is the channel imbalance the same at all volumes ? If it's worse at low volumes then it's likely a bad volume pot.
Did you try switching the pre->amp interconnects to see if it's in the pre or the amp ? IF you switch the pre->amp interconnects and speaker cables so you're using the L channel in the amp to driver the R speaker and vice-versa then that will isolate whether it's a preamp or amp issue.
Did you try switching the pre->amp interconnects to see if it's in the pre or the amp ? IF you switch the pre->amp interconnects and speaker cables so you're using the L channel in the amp to driver the R speaker and vice-versa then that will isolate whether it's a preamp or amp issue.