A lot of those old amps had (now obsolete) low noise high bandwidth Toshiba and Motorola transistors which are not easily matched with today's offering. The unavailability of those parts is the main reason why amp manufacturers refuse to work on thirty year old equipment. So an argument can be made that some of the old stuff were better than today's, but not as a general statement.
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?