Let’s simplify negative feedback = taking something ( the AC audio signal aka music ) that has ALREADY happened, flip it around out of phase ( that’s the negative part ) and feed it back into the input where something new and unequal is happening ( unless you think sine ways are music )… then apply some critical thinking….
This statement is false. Its based on the idea that there is a time delay between input and output. What actually happens is there are frequency poles in the amplifier that cause phase shift; on an oscilloscope this appears as a time deviation if you are measuring in the range of the phase shift.
Filter theory tells us that there will be phase shift at some high frequency (in an amplifier, the high frequency roll off will be on a 6dB per octave slop initially; this will impose phase shift to 1/10th the cutoff frequency). As you increase frequency, eventually the phase shift is so acute that the feedback becomes negative instead of positive, and the amp goes into oscillation. Norman Crowhurst was writing about this 65 years ago; none of this is controversial. When the amp goes into oscillation, it can be said that its phase margin has been exceeded.
http://www.tubebooks.org/Books/crowhurst_basic_3.pdf
(link above for those interested in how this really works)
This explains what is really happening and is not some sort of pseudo neo-science.
So if one has been paying attention, one should see what the problem is with feedback in traditional amplifiers (which includes all tube amps). You have an output transformer in most cases, and it causes a fair amount of phase shift, often inside the audio passband! As a result, it poses a limit on how much feedback can be used, and is a guarantee that the feedback will cause harshness and brightness as a result (I already explained why).
Futterman applied 60dB of feedback in his OTLs, but the problem he was up against was not a lot different: his design not only had significant poles in the circuit design, but he also had overall insufficient Gain Bandwidth Product. This means that feedback was decreasing at higher frequencies and so harshness was the result. Because of the frequency poles oscillation was an ever-present danger as well.
Bruno Putzeys wrote a wonderful article on feedback.
https://linearaudio.net/sites/linearaudio.net/files/volume1bp.pdf
It explains much of what I’ve been talking about here in greater detail. Don’t worry if you can’t follow the math 😁 You’ll note that he mentions Peter Baxandall’s writings of how feedback imposes distortion of its own, reflecting Norman Crowhurst’s observations (and explanations) of that from 20 years earlier. Those individuals never got the chance to play with an amp with sufficient bandwidth and loop gain! Its precisely that (sufficient bandwidth and loop gain) which is why an older amp might not keep up with a state of the art design (although it might easily keep up with new amps that are rehashed circuits from earlier decades).