Vic, you've gotten the advice of some very knowledgeable members here. Your choices are either learn enough to do your part in a remote diagnosis project (assuming one of these members or another one is willing to do it with you), or take it in for repair.
If the latter, tell us where you are so that someone can possibly recommend a remote tech.
If the former, you will need to get a decent multimeter, learn the principles of using it to measure live voltages and test transistors, diodes and transistors, and learn how to do small scale electronics soldering and desoldering.
Something more than bias, offset or dirty/oxidized contacts is most likely amiss here, By the way, I would recommend, strongly, against playing with bias and offset adjustments.
If you have a multimeter, post your DC offset measurements. With the amp off, put the black probe in the the black terminal for Speaker A, right channel. Put the red probe in the red terminal for Speaker A, right channel. Select AUX on the front panel, turn volume to zero, balance to middle, and tone controls to center. No filters on.
Now turn on the amp (make sure Speaker A is selected), and measure the DC voltage. Turn off the amp, remove the probes and re-insert them into the Speaker A left channel speaker connectors (again, red to red, black to black. Turn on the the amp and read the DC voltage on that channel.
Report back.