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

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.

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

speakermaster536 posts07-07-2021 The fine detail and harmonics is normally lost in the speaker and when you find a good one you will realize what you have been missing the whole time and keep an open mind because the good ones are not the new stuff!

I agree that speaker enclosures can be killers. Sound coming from the speakers is terrific, but the reverse sound trapped in the enclosure exerting pressure and interfering with the cone's free movement is something that really needs to be thought through.

I treat all internal surfaces to reduce reflections and use the best absorption filling I can find to convert sound waves to into heat. (Yeah, I know a lot of people rave about compromising the enclosure's physical volume and all that, but I've found it ain't necessarily so, or to put another way - necessarily critical.)

Modern cones and support mechanisms shouldn't be dismissed, so I'm not at all convinced "the good ones are not the new stuff!".

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

I don’t want gross harmonic distortion, but don’t abuse or lose the precious harmonics in the virgin recording either.

Since you are specifically addressing amplifiers, you are correct to be worried. Electromagnetic interference (EMI) is the killer and it's wise to address it as best you can, both internally and externally.

Unused ports can act like antennas with direct paths to sensitive circuits, so that's one external source easily fixed with dummy shielding plugs. Since most equipment have AC supplies, that's the internal source. If you are willing to open the case, in most cases it can be resolved by fitting a grounded shield between the power supply and amplifying circuits.

As others have rightly mentioned, amplifiers are just the start and EMI / RFI should be addressed wherever possible. Shielding power cables can stop the EMI from radiating outside of the cables, so they are well worth considering. Since RFI (Radio Frequency Interference) is everywhere around us in the atmosphere, shield everything and connect it directly to a central earthing point, not the equipment's case.

A 20Hz to 20KHz bandwidth system is an absolute necessity. Just because it provides a full bandwidth doesn't mean there the finer detail and harmonics are being lost, which I believe is the question.

The best approach is to treat your equipment in the same way as sensitive medical equipment.

Here's an interesting study.

Inaudible High-Frequency Sounds Affect Brain Activity: Hypersonic Effect
https://b01fe120-8355-4a65-8abc-c8b51b6192cc.filesusr.com/ugd/d25f4f_a07d2fffb4244c4aa3341eb3c12e1e3...