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

Amplification is a solved problem.   Class D is probably the biggest innovation.  These days, just buy the amp you like the looks of that you can afford and you will be fine.   The biggest innovation in audiophile world has been the amazing ability to cram dollars into amplifier designs.

Class D is probably the biggest innovation.

They are close but still does not surpass the best of linear Class-A or A/B designs YET!
I DID SAY YET!!! (before you Class-D junkie’s get your knickers in a knot)

That will come maybe after they take to what Technics did, 3 x higher switching frequency 1.5mhz with the SE-R1 (my last post), one of two of Class-D’s Achilles Heels, too low a "switching frequency" and subsequent low order "output filtering" to get rid of it.
Which still leaves large remnants of it (photo) in the upper mid//high frequency audio band when using around 600-700khz switching frequency as most do today.

The other Achilles Heel is current delivery into very low bass impedances of today many top very hard to drive speakers (which the "better" linear Class-A A/B amps do standing on their heads).
To be able to "almost keep doubling it’s wattage" from 8ohm to 4ohm to 2ohm with independent testing, they never give the 2ohm wattage of class-D when comparing the 8 then 4ohm

Cheers George

Better parts. Especially power supplies, volume controls. Better caps, resistors, boards. Better chassis construction and connectors. I think that consistency has also had an affect,

sound