who said you had to cut a part
your amplifier at this level is not adjustable, you have electronic problem(s) you have to differenciate ..
first you check supply lines and all the related voltages, for sure something is affecting the outputs
are you equipped to do some tests, voltages tests, component tests normally when possible at zero volume you take CAREFULL MEASUREMENTS on the good channel at key points (voltages, in and out of each tansistor chain) around the biais transistor etc ....
AND CROSSCHECK on the bad channel to see where it begun to drift, could be a simple as : a cold solder / bad connection .........