Are future improvements in Amp/PreAmps slowing to a crawl?

You were closer to understanding what I’m talking about when you refereed to propagation delay or group delay.

I did ask you about that earlier but you seemed to not respond. So is propagation delay what you are talking about? IOW, if the amp has constant propagation delay over the entire band of frequencies to be amplified, would that satisfy what you are looking for?

I agree 100%
I do not have a single device with perfect linearity because it does not (currently) exist.

The harmonics arise from unavoidable variations in gain.

Thank you - this makes my point that you can alter the pitch by varying the gain.The harmonics are the result of sliding the fundamental to a different area of the spectrum.

A change in gain is a change in velocity.
When you stabilize the velocity - you stabilize the gain.
A stable (constant) gain is linear.
A linear amplifier has no distortion.

What I have is a circuit that is 100% linear. (made from non-linear devices).

All that is needed is to use the devices in such a way as to force the output to be linear on a scale that is inconceivable.

A word about my previous comments...
I want to walk back my statements about the text books.
before you get too excited its not for the reason you think.

I’m sure that everything I have done can be found in the text books as individual or separate phenomena. What is not in the books is the composite use of various phenomena to produce a result for which there is no reference or example.

I have developed a way to take advantage of known phenomena involving aspects of how sound waves flow in air and created a circuit that treats the (sound wave) data as if it was in the acoustic environment.

This successfully "feeds" the brain in such a way as to believe these sounds are real (live) and happening in your airspace.

Question: why be so concerned about distortion of the amplifier? Didn't we find out a long time ago that some amplifiers with relatively high THD - I.e., more than an order of magnitude higher than those amps with vanishing low THD - actually sounded considerably better? 

Didn't we find out a long time ago that some amplifiers with relatively high THD - I.e., more than an order of magnitude higher than those amps with vanishing low THD - actually sounded considerably better?

Quite often, yes. The ear does not seem to care so much about lower ordered harmonics, but cares a lot about higher ordered harmonics (5th and above) as well as IMD and inharmonic distortions such as aliasing.

The latter three can be in trace amounts that are hard to measure but are easily heard by the ear (which converts them to tonality) as brightness and harshness.

Its not that hard to design an amplifier that is absent the higher ordered harmonics. Such an amp will not use any loop feedback though, as feedback will add harmonic content and IMD of its own regardless of the left/right 'speed' of the amp (as seen on an oscilloscope). See Norman Crowhurst.

SETs have been taking advantage of this for some time now (part of the reason they have such an avid following in high end audio). We do as well, but get rid of considerably more distortion merely by using fully differential balanced topology and only one stage of gain.