12 AWG Mains Power Cable From Wall To Power Conditioner Sufficient?

For power requirements purposes: Yes.

‘For highest sound quality: No.

Best use 500 MCM copper..............😉

Yes, sufficient. 

I prefer a quality insulated cable. I couldn't hurt. Just don't spend a ridiculous amount. 

Yes of course as said before if it’s on a 20 amp breaker, it’s the same  gauge as what’s in your wall. And sound quality has absolutely nothing to do with that at this point !!