Amplification: what are the biggest advances of the last 40 years?

As an audiophile most of my adult life but without any engineering expertise, I wonder how amplification has advanced since I started in this hobby as a high school student in the eighties?

Specifically, what has advanced the state of the art and what, specifically, make newer products sound "better" than older ones?

Is it that circuit design has advanced so much?  Or is the bigger difference parts quality and the technology leading to these better parts?

And please, none of the banal "it all matters" comments.  What I'm asking: which of the above matters the most?

Ralph is right, of course. The general trend is up. A rising tide lifts all boats. The ones that float, that is. Never forget that one little detail. Makes all the difference in the world.
During a recent visit at my local dealer, I inquired as to why the new amplifier we were listening to was so expensive?
An Onkyo Grand Integra M-510 (1984-1992) will cost about $13k today’s dollars.

Is it true that the best newer amps always sound better than the best older units?
Compare within the same brand/line, YES!
For example:
Pass Labs XA100 > XA100.5 > XA 100.8
Accuphase E-530 > E-560 > E-600 > E-650

"always" I can’t say...

The Eigentakt modules, the Bryston Salomie circuit, subwoofer amplifiers and speakers with built-in amps with complex DSP room correction. 
@imhififan : Especially Nelson Pass once said personally…that it’s actually the market itself that „requires“ some so called updates in his amps. He said that when asked why there is technically no difference between his X600 monos and the successor X600.5‘s …apart from a slightly different front plate. He said it winkingly … as he actually nailed the design of his monos already with the X600! The rest you can call MARKETING ;)
I think that other than refinements in circuits the biggest improvement is the GaN transistor and the new amps implementing them.