What you are saying makes sense. What measures well doesn't necessarily make it sound better. I don't have a very good understanding about electronics although I am interested and I want to learn. Let me ask you this. I have heard and read that equipment that uses chokes in the power supply sound better. I know tube equipment uses chokes and some higher end solid state equipment use chokes. The guy that builds the mystique DACs says his sauce is based on Chokes and R2R chips but it isn't a tube DAC and they get great reviews from people comparing against several other DACs that aren't reviewers. Danny has said the Schiit Tyre amp sounded suprisingly good and it has chokes in the power supply. Do they affect the negative feedback in any way?
Chokes, coupled with good capacitors, in the power supply do an excellent job of filtering out power supply noise and hum, including noise generated by the rectifier switching and transformer resonances. Most amplifiers do not use active regulation, so a power supply employing chokes can do a nice job cleaning up the signal.
SET amps have fairly poor power supply noise rejection, so good quality power supply filtering is key. Push pull amps, can cancel out a fair amount of the power supply noise, so it's a little less important here, but still helpful.
Global negative feedback will improve the power supply rejection, so is often used with lower cost amplifiers which can't afford the components necessary to do good quality filtering. Global negative feedback will also improve the amplifiers ability to maintain the correct output voltage under varying load conditions, lowering the output impedance and making the amplifier more of a voltage source instead of a current source.
But global negative feedback also introduces its own problems. This is mostly because it is much more effective at lower frequencies than higher frequencies, so it will reduce the lower order harmonics more than the higher order harmonics. This tends to make the amp sound brighter and harsher. This may sound like it is providing more detail, but it is primarily emphasizing the higher harmonics.
Low order harmonics (particularly second and third) do an excellent job of masking the higher order harmonics, so reducing their dominance will result in worse sound (in my opinion) but will also reduce the measured THD.
Many have attributed higher second and third order distortion to a more natural and spacious sound and a deeper sound stage. This is why so many audiophiles love their SET amps with no feedback (I'm one of them).
As noted above, the big downside of no global feedback with tube amps is the output impedance can be quite high (and thus low damping factor). This can make the bass sound bloated and sloppy, depending on the speakers used. Some people are ok with this, or even prefer it.
Personally, I think the best of both worlds is to use a zero-global-feedback SET amp for the upper bass on up and use an amp with a high damping factor for the lower bass.
It is possible to measure the effects of global negative feedback and it's influence on the distribution of harmonics. The best sounding amps, in my opinion, are those that have a steadily decreasing prominence of harmonics in the distortion profile (e.g. 2nd is higher than 3rd, 3rd is higher than 4th, etc.). But again, if the amplifiers damping factor is low, the speakers impedance vs frequency can greatly affect the tonal balance, and the bass is likely to be less well controlled.
