20 Year Old Amplifiers compared to 2017

Could not disagree more. Class D amps are indeed on the cutting edge and the future. Even better .... powered dacs. Powered dacs with room correction are another huge development. In the future there will be no such thing as a separate, stand alone Amplifier. They will simply be modules built into digital gear. Immense innovation has taken place. In 20 years amp modules will fit in the palm of your hand and be the norm.  

I find today's Pass amps better than Pass amps of 20 years ago

Although design can remain the same, better capacitors, faster diodes, higher quality resistors will make a difference.

There is one variable we can not quantify, and that is "hearing"; some of us can hear better than others. Although I have never heard a class D amp, I can not imagine one as good as a tube amp; however, as good as a SS amp is conceivable.

If people listened blindly, I bet they would be willing to settle for a cheaper SS amp than the one they were lusting for, but there are those with exceptional hearing.

All the nuances in music only come through in the best systems, which is why the resurgence of analog; there's nothing like the best vinyl.

In the conversations we have, there isn't much difference between, we'll say SS amps in the high end. Those who can hear the differences, are most fortunate.

I can't subscribe to the notion that all competently measuring amplifiers sound the same. Why?? Because my ears tell me differently. Way back 20 years ago a Krell amp sounded quite different from a Mark Levinson. Just recently, I had the chance to hear both Ayre and D'Agostino amplifiers in my system. Same result. They sound Significantly different. I always have my wife, as a second set of ears, confirm what I hear. Since I know what I'm changing, I give her the opportunity to hear blindly. I won't tell her what I change. I simply ask her to describe what she hears. About 80% of the time we agree on what we hear.

Amplifier technology reached maturity in the seventies, when output transformers were abandoned in solid state amps. Ever since, well designed amplifiers used within their specifications have exceeded human hearing acuity.

This statement is false.

Humans can easily hear the distortion of most modern transistor amplifiers. The brightness and hardness of solid state is well-known and a topic of endless debate- the brightness and hardness is the direct result of the fact that the human ear/brain system uses higher ordered harmonics to sense sound pressure. The slight amount of distortion is converted by the ear to a tonality. The ear has to be keenly sensitive to these harmonics because of the wide range of human hearing (140 db). So its far more sensitive to them than the lower ordered harmonics.

If anything has changed since the 1970s, its our understanding of human physiology. The audio industry has been a bit slow to respond to that knowledge though, probably because the facts are a bit of an inconvenient truth.