In the last 20-30 years, what has changed most...

I would not even mind going back even further on amp designs. There were many built at the dawn of the era of HI Fi that are still valid today.

First that come to mind are the Western Electric designs, the Marantz 16, Macintosh 225 and 275 and HK Citation.

Even the old Dynaco Mono's are still a very valid amp, even by today's standards. All of these benefit from the great tubes supplied in them from the factory, the same tubes we are scrounging around to find as NOS or clean used, even today.

Oh me God, Albert, the Western Electrics! Awesome (pls excuse the expression)! Listened to a modified such sys in Paris, running off a Detlof type Goldmund ref TT and, yes, it was more than impressive. The speakers were horn-loaded cinema gear, also WE, salvaged from some sale in the '70s.
Nothing that fits in the modern Euro-home. I wonder if it fits the average US home, even. But, by memory comparison, my sound (in a 23/13 room) is sadly compressed.

Well are Flat Screen TVs included because what has happen there is Unbelievable .The quality of the picture and the size you can have in your home and the price .While audio equipment has gone thru the Roof...Flat Screens have Dropped to Crazy Prices.85 "to 100". $2,000 .For picture quality ,the freaking people on screen are in the room with you.How is this possible  ????

Prices.