I know a while back, there were some that ended up with one or both channels inverted (flipped polarity). Most likely due to poor quality control at the transformer manufacturer; since everything's on a printed circuit board (no internal wires), it's impossible to screw up final assembly.
I have one of those with an inverted channel. When you sum to mono, it cancels.
I identified the inverted channel by playing back a sound, recording it, and comparing the waveform to the original.
Long past warranty; tried emailing Peavey and never got a response; never bothered trying to call. Really, it's the sort of manufacturing defect that should not be subject to warranty, but I didn't have my act together so there's probably not much I can expect from Peavey.
Previous discussion here:
https://forums.prosoundweb.com/index.php/topic,166398