Marantz 1060 - original amps provide the best sound. Refurbished becomes necessary at some point. I have mine driving my outdoor Klipsch rock speakers and am constantly amazed at the musical qualities it is able to produce. Tried a couple of more modern amps and they sounded flat and expressionless. What a classic treasure.
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?