20 Year Old Amplifiers compared to 2017

Just a random thought, but I’m curious just how well the state of the art solid-state amplifiers from 20 years ago compare to some of today’s better offerings. For example, what does a pair of Mark Levinson 33Hs or a Krell FPB 600 sound like if compared to the latest offerings from Pass Labs or Ayre Acoustics?
That's why I always say "If you didn't hear it blind, you didn't hear it."
That's silly. I hear my system make lovely sounds quite frequently, yet I almost never listen to it blind.

I used my mid-seventies Crown DC300a’s and DC150a’s well into the early 2000’s - typically bi-amping - sometimes with stacked cabinets like larger Advents and using an out-board xover. I used them with a wide variety of pre-amps from Threshold FET 2 and FET 10 to Mac tube. Probably not sonically wonderful but, I always loved their sheer power and ability to brush-off transient high current needs. In the late 90’s I had the 300’s re-capped and new output devices installed. I just pulled 2 pairs of Advents out of storage and began using them as the fronts in my surround system (Yamaha RX-V3800) and was thinking "I sure wish I still had those Crowns" But then, I'm a sucker for nostalgia.
I have a thirty year old Threshold 400A amp and a twenty year old Threshold T2 preamp that have been upgraded with new caps, transistors and binding posts and given their age still sound wonderful. Over the years I have gone through several sets of speakers, CD players, DAC's and cables but have hung on to the Thresholds.
Indeed, willemj is right.

And so was Peter Aczel, of The Audio Critic, when he said: “Longtime readers of The Audio Critic know the drill that comes at this point: I repeat, for the nth time, that all amplifiers having high input impedance, low output impedance, flat frequency response, low distortion, and low noise floor sound exactly the same when operated at matched levels and not clipped. (Those who are unable to stomach this simple truth, proved over and over again in double-blind listening tests, should stick with Stereophile.)”

Of course, it's true that certain unique loudspeakers can present loading that will cause good amplifiers to sound slightly different; refer E. Brad Meyer's article that originally appeared in the June 1991 issue of Stereo Review. (More recently reprinted in the Boston Audio Society's monthly BAS Speaker publication.)

And while it may be theoretically possible to construct a vacuum tube-type power amplifier that will initially approach equivalent excellence, that product will also exhibit persistent and continuous degradation from the time of initial turn-on until its ultimate end in cathode depletion failure—barring other modes of premature demise (e.g. open filament, vacuum leaks, gassing, microphonics, atypical distortion, hum/noise). So vacuum tubes are not a wise choice when stable, long term circuit performance is a serious design goal.