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

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.

@pedrob - thanks for the advice. I've always been curious what an extra grounding system would do, for instance. And some shielding plugs has also been recommended. I like that idea too. 

To get high S/N and low THD you need to add quite bit of dB of negative feedback to the circuit. 
This reduces the second harmonic and increases IMD…leaving you with a harsh and sterile sound, stripped of harmonics.  
So basically what I’m saying is you may need a poor measuring tube amp if you want harmonic richness without transistor distortion. 

My recent foray into digital coax cables includes Pangea Premier SE 1.0m, 1.5m, and finally the 1.5m XL, which is taking a good bit longer to break in, but provides a more intimate presentation, after 3 days with lots of replay (~20 hours so far) still missing a bit of HF sparkle, but I am confident it will come around. Best $150 I've spent in a long time. Are more expensive coax cable that much better. I doubt Ill be finding out any time soon 


hth

I don’t want gross harmonic distortion, but don’t abuse or lose the precious harmonics in the virgin recording either. No way to measure that, though, right?

Sure you can.

The thing is, no amp is going to lose harmonic information in the audio range. They will all *add* harmonic information though and because distortion obscures detail, it will subjectively appear that some amps are better at getting the 'harmonic structure' (overtones) right than others.