Thank you very much Chris! Tomorrow evening I have free time and will check theses. Will let you know later. Good night! (in my local zone)This amp has two important gain-setting feedbacks. The first is the input diff stage's degeneration resistor (10K Ohm). These need to be pretty close to the same for each channel. The second is the ratio of the three-in-series 39K resistors to the 3K9 cathode resistors that they're feeding. These need to be as close as possible to "the same" for all four ratios. I'd look hard at the 39K resistors because they've been running the warmest.
All good fortune,