@toddalin Maybe that's part of it (volume). I do know that during longer listening sessions my tolerance and desire for higher volume increases.
I do think this 'midnight effect' exists and I'm resigned to believing it's an external power phenomenon, probably some form of electrical noise. I still don't understand how it can pass through something like a PS Audio P10, which, as I understand it, converts the incoming AC into DC, then regenerates the DC back into a precise AC waveform. I would've thought that acts as a big filter and that line noise would be eliminated. Maybe someone should tag Paul McGowan at PS Audio.