This is a comment on Lynn Olson's comment. Decades ago I had a demo by David Berning and Murray Zelligman at CES 1979 of the Audionics BA150 amp designed by David Berning. The amp had variable negative feed back from 16 dB to 0 dB, As the feedback was reduced, of course the harmonic and IM distortion increased and the frequency response got lumpier due to increased output impedance. But the sound got better, more ope and more dynamic and more life like. I asked them bothif they ever measured anything that correlated with good sound since the classic measurements were a poor correlation expecting 'no'; you just have to listen and fine tune by ear. But the reply was yes, simple linearity. If 1 volt in gives 5 out the 2 volts should produce 10 out, etc. And the better a device did this, the better the sound.
Halcro update
Maybe old news for some, but I just chanced upon this article that helped clear up some things about Halcro that I hadn’t known. I remember back in 02 and 03 how they were the big and new and exciting kid on the block and then they vanished and now I know why. I’ve never heard or had experience with her equipment, but their design was quite interesting and knowing the physics behind it, or at least the physicist makes it even more intriguing.