I used to think THD is not important until I research into speaker cables. THD is everything. I think people that claim THD is not important likely to have inferior speaker cable. I have a long thread on speaker cable and distortion. Speaker cable is everything. 10ft of Monster cable raise the THD from 0.006% to 0.14%. If you don't have a good cable, you'll never get the benefit of an ultra low distortion amp.
I might be new in audiophile, But I have been a senior engineer and manager of engineering for 30 years designing with transistors, IC design and all different things. I also designed tube guitar amps with channel switching, power scaling. So please don't talk to me as if I am a green horn. I owned two US patents solely under my name and I published papers in America Institute of Physics Review of Scientific Instruments.
There is a lot more to electronics than just by try and experience. You can have 30 years of experience OR you can have 3 years experience repeating 10 times. Theory is everything, you have to have solid grounds to stand on. No GNFB is ONLY your opinion, there are plenty of people disagree with you. All I know is My amp has better clarity, more separation and better sound stage than the Famous Stasis amp designed by Nelson Pass that has no GNFB. AND it is a quite expensive amp. I am not going to say I know it all as I don't. But I did accomplish a big mile stone on my very first design.....based on theory and surprisingly, proofed with listening. My amp comes alive.
Years of experience in ultra low noise, microwave and HV design taught me to conclude that those who think it's black magic is very likely they don't know enough to explain it. You work long enough with so many PhDs like me, you'll see the other side.....that EVERYTHING can be explained IF you know enough. I designed RF circuits of multi GHz from paper and simulation to real circuit ONE TIME THROUGH with performance, Smith Chart plot almost identical to my simulation. It's the knowledge that matter. If you cannot explain it out, you just don't know enough.
EDIT:
Back to the speaker cable, I have a long thread on testing, comparing and building speaker cable and the difference in sound due to speaker cable.
http://audiokarma.org/forums/index.php?threads/effect-of-speaker-cable-on-thd.764633/
The different in changing the speaker cable is staggering. With the old multiple monster 12 gauge cable, I could not tell the difference between the Stasis amp and my amp. BUT with the new cables I made, the difference came out clearly. It's all scientific, and then proof by listening.