Why do audiophiles shun feedback in amplifiers?

since IM will occur at the feedback nodes of the amplifier, resulting in a harmonic noise floor (rather than a natural hiss noise floor caused by component noise).

Atmasphere, I'm not sure I understand it. IM is caused by the presence of two frequencies on non-linear element resulting in "harmonic noise floor" meaning that there will be new frequencies of small amplitude harmonically related (sum and the difference) to original frequencies. NFB reduces IM by linearizing circuit - therefore REDUCES harmonic noise floor. Very deep NFB can make it almost perfect practically eliminating IM at the cost of introducing TIM. Eliminating NFB might be possible with the inherently linear tubes but not with transistors that require linearization and regulation.

Interaction with the speaker might have more to do with the type of the amplifier - SS representing Voltage Source and Tube amp representing Power Source.

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