I agree with oldhvymec
I’m not buying into “It’s the amp”. If you set it up one way and the amp works perfectly, and then change the set up and you have a problem; it’s probably the set up, not the amp. From your test result from switching the inputs your bad channel is telling you, “I”m showing you I can provide perfectly good, balanced music if I”m given the proper input”. That is enough to tell me your amp is working as it should.
My suspicion is it’s either something upstream, or the interconnect cord and/or the XLR input plug on your amp. Could be an intermittent soldering fault in your interconnect or XLR connectors that cures itself when positioned slightly different.
When you have the inputs reversed at the amp and the amp is all balanced and the meters match, have you tried leaving the connections reversed at the amp and then reversing the interconnect connections on the upstream component (while not moving interconnected wires any more than necessary)? If your VU meter levels still match you would now have left music coming out of left channel and right music coming out of the right. At the very least you would have good music to listen to while you try to figure out where the problem is. Meanwhile, with both your left and right channels playing together as they should, you can be 90% assured it ain’t the amp. (Still could be a bad connection or part in or around the input section).
I’m not familiar with your equipment. Do your amp and your upstream component both have RCA connections as well? If so, you might try using the RCA route to see how the amp works. If the problem disappears the amp is fine. If the L/R problem remains, it is the amp.
Just some thoughts from a guy who has many components tied together in his system and occasionally has to “find the problem”.
Good luck,
Tim