I think we all owe a debt of gratitude to Mr. Nelson Pass.
Historically, he has given us audiophiles some of the greatest SS power amps. 2nd would be Mr. Mark Levinson.
Honorable mention to Bob Carver & John Curl.
I'm sorry Jafant but i think you are very ill-informed. An honorable mention to Bob Carver & John Curl?? Are you kidding? Are you serious?? The Mark Levinson amps that you have been enjoying are all John Curl designs. Mark Levinson was a ditz & just the marketing head of his own company (when he ran it) & it was all John Curl designs that were badged Mark Levinson & sold. Some of those older amps still sound superb today. Not to mention that all Parasound amps & the Halo editions are all John Curl designs.
And, as far as Bob Carver is concerned, you can read up on just how much pioneering work he has done in the power amplifier space. You might not like his amps sonically but that's another matter altogether.
In tube amps while Audio Research's William Z. Johnson did some pioneering work (I've read his 1968 patent that deals with partial cathode coupling) towards tube amp development, significant work in tube amp arena was done by those people who made OTL amps viable & reliable. These are people who started NYAL & Atma-sphere (I don't own any of his amps or preamps) & Futterman.
And, before I tip my hat to Audio Rearch & CJ I'd be tipping it before to people like David Hafler who designed tube amp kits to bring tube amplification to the masses so to say.
I really haven't seen a major technological improvement in circuit design. regulators, power supply, circuit topology, etc. no real major changes. Just better components, layouts, etc. And as I wrote earlier, better transistors.
This is also total BS!
There have been great strides in power supply design esp. in making them less noisy, wider bandwidth & having better line & load regulation (like Kijanki wrote before). Additionally, there are a very few power amps that have a regulated linear power supply. This is a very hard thing to do given that it has to be high current & high bandwidth. AFAIK, this design is patented.
Even tho' transistors have gotten better, if you give better parts into the hands of a mediocre or shitty designer, you are not going to get a better power amplifier.
We've seen better power amplifier topologies materialize such as those that use very little global negative feedback & restrict almost all the feedback to local negative feedback. This one aspect of the design has vastly improved today's power amps sonically compared to yester years. Anyone who does not acknowledge this has his/her head buried in the sand.
We've also seen much better thermal management techniques using both electrical circuits to detect temperature rise & quickly deal with it & physical hardware to wick away the heat generated.
We've also seen power amp designers get a much better understanding of circuit theory w.r.t. what makes a power amp sound better & they have been able to use this better understanding to use higher quality parts where it matters most (rather than using high quality parts everywhere & racking up the cost). So, in this case it's a better execution which can be considered as an advance in power amp technology.
We've seen a significant improvement in power amp reliability i.e. much longer MTBFs esp. for solid-state amps. (given that the user doesn't hook up the power amp in some stupid config & blow it up). Even tube power amps have come a long, long way in this regards. For the most part if a tube amp fails the damage is localized & often can be repaired by the owner.