I believe that improvements over the last 20+ years, and still ongoing today, differ depending on the type of amplifier. Solid state has changed significantly, as pointed out by @ghdprentice using Pass as an example. However, changes in SET amplifiers (300B, etc), for example, has changed much less. Yes, there have been improvements in capacitors and transformers, but essentially the sound of a 300B amp today is not significantly different than one 20 years ago.
Maximum Upgrade Potential
I have a 20+ year old Prima Luna Dialogue One, and am looking at other tube amps for some unknown time in the future. I recently replaced my power tubes, and a thought occurred to me: When do you get to the limits of an amplifiers ability to be improved, or maximum sound quality? If I got the best available tubes, power conditioner, cables, etc., how much improvement would I expect relative to an amplifier a step or two up the quality chain? Will the best and most expensive tubes improve all amps equally or benefit some more than others? And at what point is one just putting lipstick on a pig?
Thanks,
John Cotner
New Ulm, MN