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

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

Try not to worry so much.
Loads more is lost in other parts of the chain.

As you rightly say OP, 120dB below the signal is a looonnngggg way down.  You can demonstrate that to yourself if you have an amp with numbered volume settings at known dB intervals.  You won't hear it, but the full volume signal will blow your eardrums if you're silly enough.

@audioman58: the venerable Quad 57 can still be considered a reference speaker! They can usually be found for around $2K. I have two pairs.