Why do audiophiles shun feedback in amplifiers?

Atmasphere,
Your explanation of the natural noise floor compared to the 'harmonic noise floor' sounds absolutely right,here`s why. I know my former Symphonic Line and my PP tube amplifiers will measure lower distortion than the 300b SET amp. Yet when I listen to familiar music with these amps the SET in reality had the lowest noise floor for actual listening. It teveals more nuance,inner detail,ambience clues, all the subtle sounds that were either buried or not heard at all with the other amplifiers.This contrast is very apparent. Thanks for this explanation. I could hear all of this easily but did`nt understand why.This is likely a major factor why evrything sounds substantially more real and convincing with the DHT SET in place.It all makes more sense now.
Regards,

Ralph, by "tightness" I'm referencing real acoustic bass instruments such as kick drum or stand up bass. Therefore, it is not an system artifact.

Getting back to my original point...here's a thought experiment. Let's say you design a tube amp with different levels of negative feedback, let's say in increments of 3 dB from 0 to 30 dB. Assuming you have a good speaker/amp match, what you you hear as you increase the negative feedback from 0 to 30 dB? This may be simplistic, but has someone actually performed this kind of experiment?

Dracule1, Read this:

http://www.stereophile.com/reference/70/index.html

Dracule1, I have played string bass since 7th grade. 'Tight' is one thing is isn't. Energetic- yes, detail- sure- its a real instrument. But you won't ever find one sounding 'tight'. We may have a semantic problem here; for me 'tight' is punch but little else. Sure, I want the impact, but I want the detail too, and that is something that lots of feedback robs from the bass frequencies- things 'stop' too quickly. IMO/IME its the bass detail and ambiance that goes away first as things go wrong in a stereo.

Kijanki, I would agree with you regarding what NFB is **supposed** to do, but if you also have propagation delay in the amplifier there is no way that the NFB is not mixing with a different frequency- thus the IM. You might want to read Norman Crowhurst- he mentioned this very issue in some depth about 55 years ago.

In a nutshell, low IM is a function of linearity in the various circuits of the amp. If there are non-linearities and NFB is applied, its not reasonable to expect that there will be no IM afterwards. Instead, while the IM will appear to go down, you will find that the energy of the distortion is spread out over the spectrum- that is to say it is by no means eliminated.

Chaos Theory does apply here. If you analyze an amplifier operating with NFB it basically is a chaotic system, complete with bifurcation (which we audiophiles call distortion) and a strange attractor (which interestingly, Norman Crowhurst graphed before Choas Theory was a recognized science!). The formula for NFB and a classic Choatic system are strikingly similar, if not identical.

When you use Choas Theory to analyze an amplifier, then it is easy to see how IM and NFB interact. Imagine a balloon on the floor with air in it, and then a weight placed on top of it; the balloon will squish out to hold the air before it bursts. An amplifier with NFB is similar- when you look at the open loop spectra, using NFB is like adding the weight to the balloon. The spectra expands across many harmonics, with inharmonic information added due to intermodulations at the feedback node (I am nearly quoting Crowhurst verbatim here but that is the succinct way of putting it). IOW the energy of the distortion does not go away nearly so much as we have been led to imagine in many school classes!