I've been playing with some vintage Sansui professional series ( BA 2000, CA 2000) pre-power for a year or so as a hobby, and i have to say after a re-cap,etc they are surprisingly nice sounding. Better then the Schiit Aegir in bass, midrange detail and decay only place the new stuff is maybe better is high end extension but that's debatable. so I'd say if you find a quality item form the vintage period it potentially can be stellar if updated-recapped as necessary etc. The key to most vintage stuff I've come across is the fact its aging and if not addressed they are most likely not close to original spec as parts drift with age. once in spec much of the stuff can be quite good. specially the tube gear if it had good transformers to start with. of course your mileage may very and its obviously dependent on what vintage item your looking at, as like today not all gear is worth looking at.
Old Amps that can still Kick Butt
Not being a believer that time necessarily = progress, I would like to offer the following example of a sonic gem that has transcended time and can totally kick butt in a modern milieu:
The Robertson 4010. I got one of these about two years ago because it was in immaculate condition, the price was so low and I was inquisitive. I hooked it up and let it warm up for a couple of days. OMG this thing was in the super amp league: Transparency to die for, slam that you couldn‘t‘ believe for for a 50W amp.. Peter Moncrieffe wasn‘t wrong in his review of this amp: this thing is in the Sterreophile Class A component category hands down. Even after all these years.
What amps have you encountered that have defied time and can still kick butt today?