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

Trying to tie product quality and value to you to market prices is a very bad place to be. You are essentially trying to build up your stereo based on perceived fiscal value instead of pleasure you personally derive from it. Unless you are running a used gear store I think this approach won’t help you enjoy music very much.

Consider that a brand name can make a difference in selling price from 2x on upwards but that’s no guarantee you would like it 2x better. You might also fall in love with something older, or less expensive. Will you buy the less expensive gear you like more?

I've done the Vintage audio thing for some time and new audio as well. what i can say is its dependent on the specific item your looking at in vintage. Not all are as good as others. Also the technology of class A, A/B hasn't changed all that much in 40 years obviously the newer classes are a change i.e. class D etc.

The usual question is what has the older piece had done to it for maintenance over the years. All components drift out of spec over time so if that component has been refreshed/recapped etc. then yes the item can compete fairly well with modern designs of similar class type. Specially if the original specs were excellent to start with. Material science has come a long way in the last 40 years though so that has to be taken into consideration as well.  

I have older quality vintage pieces that compete very well with modern equipment. 

@knotscott … “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.”

+1