If you don't mind my asking, why the 20 year cutoff date?
I have an ARC SP11 mk2 (1987) that sounds lovely and will likely keep sounding so for decades to come. Prior to that was an SP3-A1 from 1975 that many will agree still sounds wonderful and can be easily refreshed with new caps and tubes.
My point, is why buy a new preamp/design when you're looking for older sound?
That'd be like buying a modern car then retrofitting it with a hand crankstarter and narrow wheels for a 'vintage experience'. :)
Don't write off vintage equipment...
Happy listening.