idk how to test ca stuff i fixed guitar amps over the summer. one way to isolate it under power is removing and replacing transistors 1 at a time and then 2 then 3 if needed to find the bad apple. or check current over the three legs, find a schematic of how the insides are supposed to work and if your getting current flow out of the wrong legs replace that one. thats all i know.
if one channel works fine there is no reason to replace the outputs in it. the distortion you are hearing could be a few things. it could be some bad IC's in the preamp section or even some bad caps in the preamp section. i had an old orion making terrible noise in one channel and on a hunch i changed a small very inexpensive capacitor and it played loud and clear. could also be that you have some leaky output transistors in the bad channel causing your distortion. they may not be blown to the point where they are completely shorted(which would almost certainly blow the amps fuses upon applying remote turn on voltage) but they could still not be up to spec. you can always take your meter and compare resistance readings and or voltages from the good channel to the bad channel and hopefully find something like that.
