Let me start with my cards, and philosophy, on the table. No one every bought a Stradivarius, or a Rembrandt or a Chateau Margaux based on measurements, and I'm not basing my listening experience on them either. JMO/FWIW/YMMV and all that.

Here's the thing with feedback. Feedback is used to correct other distortions, so I don't think you can't can separate what feedback hurts, from what feedback helps. If you take it away to remove one problem you are likely causing a much bigger one. And I don't think there are any measurements for soundstage or detail or warmth that will tell you one amp "sounds" better than the other and those are the kinds of things feedback affects. There is slew rate, which measures how fast an amplifier output reacts to it's input, that points in the direction of what you're asking. I used to know more about it 40+ years ago when I was taking a class, but generally any well designed amplifier is going to have a slew rate higher, likely much higher than the signal it will be amplifying. So, in theory, feedback shouldn't be an issue, but it is. A square wave is a really good test for an amplifier because it is an infinite sum of increasing frequencies, and if the ratio of the frequencies change, the output will be less square. And for a well designed amp, square wave response improves as you apply feedback properly. That's why feedback is a thing.  

Back to the Stradivarius, lots of people have opinions on why they sound the way they do, but I have yet to see any measurements that are useful as far as recreating what it does. I don't think this is any different than not having a measurement to show one e.g. interconnect or vacuum tube or capacitor sounds better than another. People have been looking for "useful" measurements to tell what sounds good for as long as there have been amps. But designers are still relying on their ears. Which isn't to say some measurements aren't useful, because everyone uses an oscilloscope to be sure the amp is in the ballpark. But at the end of the day it is Steve's ears and not his test gear that is why we all love Decware.

I'm a (recovering) physicist by training and temperament, and I believe that there exists first principles explanations for every natural phenomena. But I also know a lot of stuff in real life is really, really complicated, and for us puny humans the deepest "why" for many things is going to be out of reach for a while. Or put another way (the way I prefer), music is like gossamer, and one doesn't dissect gossamer.

I don't know if that helps or not.