20 Year Old Amplifiers compared to 2017

I doubt the "better offerings" sound much different in most cases  though no two cases are probably exactly alike due to all the variables at play.

Major advances in amp technology like Class D amps are the big difference. These put more offerings in that "better offerings" class that are much smaller, much more efficient and often lower cost. These things together make them more practical for many more than the better amps of the past.  I know it has for me.

willemj is right.

besides Class D (which allows less expensive and lower power amps), Benchmark has licensed some exciting technology in just the last couple of years which greatly reduces crossover distortion (so you don't have to try and run in Class A all the time)

atmasphere did not note that we had tubes back in the '70s....
- which burn a lot of power

I am using a pair of McIntosh MC2205's for my bi-amlified speakers, these are circa 1977 if I am not mistaken, 200W with autoformers, everything is original, and they are still running. My speakers are 96dB and in bi-amplifed mode I usually can't go above 20W per channel for each amp in a very large room, this helps keep the amps relatively cool.
My only problem with these are the weight, at 85 pounds each they are too heavy for my back.

I think upgraded old amps sound as good as new ones. Regarding the future direction of amps, my prediction is the biggest innovation will be the widespread use of battery power as cars and phones drive the development of battery power.