"They simply don't make them like this anymore"

The thing is: the output transistors that are available now are much more linear and faster than those of past years, not to mention the superior capacitors of all types that you can get now. Some of these end up inside newer amps. Same designs, better parts.

To me the value in vintage quality gear is in knowing the items to seek, understand they will most likely require service due to their age, and avoid spending really big bucks on comparable new products.

A Threshold SA3 or Krell KSA50 or vintage Kondo tube amp is a great piece. They are especially attractive when compared to the new prices. A used Ongaku can cost around 20k; servicing/upgrades will add around 5000 to that cost. A new similar product from Kondo San starts at 40 large. That is the motivation; same thing applies for something like a Levinson 27.5 vs a new/used current model Levinson.

With the rare exception of collectors it is the price that matters. When the older pieces are serviced/upgraded they are very close to most comparable modern replacement units.

Stick good Bilsteins and quality wheels/tires/sway bars on a 1970 Chevelle SS350 and it will handle right up there with a modern similar sized car. Same principle.

Let me elaborate:
Audioengr,
You hit the nail on right on the head. I own several Pass Aleph amps. I'm a fan of MOSFETS & "tube sound". I never liked the Pass X series amps enough to transition to them.
The new X .5 series amps are much better than the originals!
Everyone who has heard my local dealer's demo X 350.5 agrees on this - it is noticeably more musical than the original X 350. The new plastic cased transistors are just plain better, in my opinion.
When Sunfire came out with the Signature Sub, it was a giant leap forward compared to my JBL B380, much smaller & it moved 3 times as much air.
I see ads here on Audiogon saying "Amp X is just better than anything made now or in the past", and I wonder how many newbies are going to fall for this.
Most new solid state amps are better than their predecessors, that goes for most brands......

DanielK141 - now if they would just use the right capacitors and do the wiring harnesses right....