I’m also worried about how much of the harmonics in the music is LOST by the amp?

If we can’t measure the musical information lost throughout the chain, the measurements we do have about what gets added in terms of distortion are a smokescreen.  If you lose 25%, but add negligible distortion, you pretty much still suck.  Right?

I mean, signal to noise ratio would seem to be the most relevant stat in common usage, right?

I mean, signal to noise ratio would seem to be the most relevant stat in common usage, right?

Even when it's below audible levels, I have always wondered if it can contaminate the sound.

A very strong SNR of 120dB is almost certainly beyond audible range, but could be important for the fragile harmonics.  I’m sure there are people here who know this scientifically.

Exactly! Where does the noise come from? It it mostly ripple on the DC supply? Is a massively over engineered supply helpful?