Thanks guys. I've hooked up the m22's and had the same result! Plus, I did the battery test, and everything checked out. I can now verify with 100% certainty that it is NOT the speakers.

I never had this problem with the amplifier before. The only thing that has changed is that I replaced the RCA interconnects between Pre/Pro and Amp with balanced XLRs. Could they be the culprit? I can't imagine there's a way a single XLR cable could invert polarity.