I suppose that given this old vs. new conundrum, manufacturers would be best advised to... switch to solid state.

The manufacturing techniques might have changed. Why do the new violins not sound "as good" as the ones made by Stradivarius?
Oh and why the blazes can't we produce a composer that can measure up to Bach, Mozart, Beethoven, Schubert, or Mahler?
The arc of culture in time does not necessarily point upwards, etc.
So modernity may not be better, but it's what we got. Love the one you're with.
If you can get a good NOS tube and it makes the AVA dac/preamp/amp sound great, good for you. If you can't be bothered, don't worry, the new tubes will also sound great. Some tubes should make more of a difference than others and if you're not pushing the tube much at all then maybe they shouldn't make much of a difference at all. If you have nothing better to do than the swap tubes in and out then that's your current lot in life; if you do have something better to do then I hope that's a good thing, for some sadly it isn't.
Psychoacoustic effects are inevitable, a/b testing is ridiculous (you have to wait at least half an hour between testing sessions, realistically only Frank can be expected to do it since he can have several "identical" devices to try simultaneously), etc.
If you are consistent about demanding objective a/b testing then you might as well jump off a bridge because your parents sure did not follow that rule when your conception took place. If you are consistent with insisting that NOS tubes sound better then you might as well jump off a bridge because tomorrow will be worse than today.
Throughly enjoying Blondie's "Die Young Stay Pretty" at the moment and if we all agree that it was just a bloody coincidence that it came on as I started this then I'll be on my way.