SET amp w/Neg feedback?

Hi!
I believe it depends more on the kind of SET amp and how tough is the load (loudspeaker effieciency, impedance and phase angle).
Most DHT SET triode amps have low distortion levels and in general sound quite alright without NFB, when matched to SET-friendly speakers.
However, SET amps based on output pentodes or tetrodes usually require a modicum of feedback to reduce distortion.
If you want to hear the effect of NFB, Cary's 805 SET amp has variable feedback. I have learned that with speakers that present a difficult load, 6 db of NFB is a good thing.
In summary, as with most things in life, there are no definitive answers set in stone, one must experiment and find out what is the best compromise.
Enjoy the music

Almarro 318B has 6db of feedback.
The best bass I have ever heard from a SET.As reviewers have said it sounds far more like a 100 watt SS amp in terms of bass control.
A superb sounding amp in all other areas too.

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.