I'd record just at least a snippet from the main outs, just to make sure...

Quite a lot of real sounds from real instruments exhibit non-symmetrical waveforms, only synths are perfect!

But I would get someone else with a 3k to confirm it for you. You MIGHT have had a component failure on one of the op-amps. I'd also double check your recording signal path. Have you eliminated other sources from the same problem? Your A/D card might be damaged, your mixer's op amps might be faulty, there's a whole signal path to troubleshoot (sorry if you already have)...

Try a recording of just ONE sound, say a synth pure tone recording (sawtooth or square wave) and see if it is those too, or just the result of non symmetrical acoustic waves...

Hpe this helps.