SET amp w/Neg feedback?

The Art Audio Quartets (845 PP) have a switch for 0 or 6 db of feedback. I was trying them on my Maggies and preferred the feedback initially. One day I had some "girl with guitar" music on and went back to the 0 setting, which was preferred. It was not a huge difference but nice to have the option. Bass was better with, midrange better without.

It's what you said in your last sentence, "just about the sound". Don't get caught up in marketing hype.

There are many ways to make great sounding amps, and between theory and listening there can be a great gap. Like Onemug says, don't get caught up in what should or shouldn't sound better based on topology or techincal specs, just listen.

Using or not using feedback is NOT just about marketing hype.It fundamentally changes the damping factor of an amplifier.
Most speakers will not work well with zero negative feedback ampifiers.This is especially true for speakers with low impedences-below 8 ohms[most modern speakers].This is an electrical fact.Adding some feedback increases damping factor and this in conjunction with using tubes able to deliver plenty of current[not 300Bs] will seriously improve bass control.
Of course some people like the looose and bloomy bass of zero negative feed back SETs-but it is not accurate.

Jtgofish, that is not entirely true, although there is a class of loudspeakers for which it probably is. see:

http://www.atma-sphere.com/papers/paradigm_paper2.html

for more information.

Negative feedback violates one of the more important rules of human hearing by enhancing the odd-ordered harmonics used by the human ear as loudness cues. Hundredths of a percent is quite audible- bright, hard, harsh, clinical, chalky... -these are all words to describe extremely slight amounts of odd-ordered harmonic distortion.

Atmashere,
I am sure what you say is theoretically correct.
I have had the opportunity to use amplifiers with variable feedback.Experimenting with the amount of feedback is very interesting.Certainly too much detracts from the sound.Most noticeable is the collapse in depth and precision of soundstage when too much is applied.
However to my ears a small amount of feedback gave a much more controlled and natural sound than no feedback.
P.R.A.T. was much better.