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

There are better parts and a lot more of them on the Boutique side.

Class D amps.

The greater usage of BIG valves. The 300b is being used everywhere.

A lot of development in bigger power valves, KT90-170.

AND top Top TOP of the line amps. Valve and SS amps are just art work inside, a lot craftsmanship..

Sure are more than a few I'd like to test drive if nothing else..

Yup thing have change. I think for better and a LOT of them run a lot cooler, valve amps like the Crimson 275 and all Class ds.

Regards

From the perspective of an amplifier designer, these are the biggest improvements I've been aware of.

OTLs became a lot more reliable- as reliable as any other tube amp. OTLs offer transparency like no other tube amp.
Class D amps have been refined to the point where they are contenders. If you have one that is self-oscillating, they can have a distortion signature that lacks the brightness and harshness of regular class AB solid state amps. This means they can sound smooth like a good tube amp until you run it out of gas. I'd say that's a major break-thru.
But the ability to get rid of that solid state brightness is not limited to class D. Newer semiconductors are around now that didn't exist in the early 1980s or before, making it possible to build a class AB amp that has distortion so low that they don't sound bright. The trick in both the case of the class D and the latter amps I've mentioned here is to have feedback in excess of 35dB. This allows the amp to have consistently low distortion numbers at all frequencies rather that just at low frequencies like amps of the 70s and 80s.

So that's a big deal too.

Better understanding of the need for good power supplies and the application thereof. Also, good transformers have become more commonly available, if not used more.

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?

Please people, read the OP. Thank you.

More efficient amplifiers ie Class D and similar. Also the ability to control noise with same. That’s the big one by a huge margin that makes things possible that were not prior including beefier amps in a smaller more cost effective  package. Integrated amp technology benefits as well.