20 Year Old Amplifiers compared to 2017

Older amplifers should be refurbished, of course. The first thing that has to be replaced would be the power supply caps, as these do degrade and can fail wtih nasty consequences. As I said earlier, with luck you can fit larger modern ones as they tend to be more compact for the same value. If the amplifier has been designed competently a slow ageing of the other components should not degrade sound, or not much.
I am currently using two refurbished Quad amplifiers (a 606-2 in the main system and a 405-2 in my study). The service engineer simply replaced all resistors and capacitors, arguing that these days quality components cost very little and labour is expensive. Why test a resistor if a new one costs peanuts?

Dan D'Agostino, in his Stereophile interview, said, point blank, that some of his Krell offerings measured better than his current stuff. He specifically chose sound quality over measurements in his highly acclaimed breed of new Momentum components.

Chasing measurements will give you that clinical, dry, and lean sound. My guess is that its the sound signature of too much NFB. 

I've tried several times to replace my 20 year old amps with something comparable in price... only to be disappointed.  I own 2 Brystons... I just had my 4B-ST restored (by Bryston... an amazing deal!) and am very happy to have it working again... sounding possibly better than new with modern caps.  I also have a newer 4B-SST2... which at twice the price, does not sound better in my opinion.  It's more refined, but less exciting.  My all time favorite amp (don't laugh) is my Nakamichi PA-7AmkII.  I had it restored/recapped and it sounded better than ever for a few years, but it's now nickel & diming me with weird power up issues.  I so LOVE this amp, but wonder how much i can afford to keep it running reliably.  This is the problem with big vintage amps.  No matter how great, they can start to develop weird little problems.   I also share with the poster above... that my trusty backup amp for 20 years is a Hafler P3000.... not GREAT.. but surprisingly good!!  

Like Lancelock I have an old Mirror Image Amp a very obscure company that only lasted a flash. I recently bought a first watt F3 amp and there was no comparison between the two on my Sunfire Ribbons. Of course the older amp would be close to $10 000 in todays dollars. having tried some Class D there is something to be said to good old Class A amps. especially because it is 1 degree F today.