So same output level and same program material? how old are the amps. I bought a used sub amp once from a guy that used in db races and it worked for about 7 months and then it failed and when i oopened it , it smelled really bad and couple of op amps were burned. Electronics deteriorate over time due to heat that goes unnoticed. I am just throwing around ideas here but maybe pop the top off the offending amp and give it the smell test. I fixed my plasma tv a couple times by sniffing around.

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