How does one weigh the virtues an older amp vs. a younger one?

My experience has been that for a given level of amp, 10 years and new is substantially better than the older. Not just noticeable, but wow, that is really better. I have been at this 50 years and owned multiple generations of Pass amps from the Threshold s500 back in 1980.

A couple years ago moved over to Audio Research tubed amps. I will not be going back.

Think of an old car with few miles vs a lot of miles. No comparison. Amps are similar, if you can get an old amp that wasn’t used a lot vs was played hard, it makes a difference. I like my Carver 505 that was built in the nineties a lot and I would put it up against any of todays amps up to $1500.

OP, since you are talking integrated amps, how about a Mark Levinson No. 383.  George Meyer in Los Angeles, who is an authorized Mark Levinson repair facility has completely refurbished units at $3,000 dollars and another for $3,200 dollars.  These are outstanding integrated amps.  Here is the info:

Better to listen to each for extended periods (weeks even), then decide which one you prefer to live with.   You miss a lot of the little stuff doing quick A/B listening.

Yes, truths reveal themselves -or are built into clarity, in our minds- over time.

Audio is no exception.

A saying that goes in that direction, is that we know all we need to know about a person we meet, in the first few minutes, but it takes 30 years of interactions for that to be clear in our minds.

Human interactions with the world, in all their myriad ways, are, of course, some variant of that. No matter how much the ego wants it to be different.