I agree, there's a point of diminishing returns, and we're talking subtle details with much of this, though there are times that an NOS tube will make a very noticeable difference - it certainly was noticeable with my HXDA, and made the amp sound better.
Here's one good thing: a good preamp tube lasts a very long time, even when you use it every day. Assuming you get a good NOS tube, it'll probably last as long as the amp - which is why it pays to have the tube tested for noise and microphonics. Obviously, even good preamp tubes can develop problems, but they're more unusual with NOS than new production.
As an example, when I sold my '67 Blackface Fender Bassman, I'd used it hard for years, and after more than a decade, it still had the original preamp tubes - - and the original power tubes! And it still sounded great (I was lucky with the power tubes).
Output tubes wear much, much faster. Chances are you're re-tubing your preamp tubes because you want to vary the sound, or because one of the stock tubes went bad?
The V1 is the tube position that has the most effect on the tone. Keep your good tubes, and put the best ones in the other positions, and just replace the V1 and you'll maximize your dough. Modern preamp tubes tested for noise and microphonics are not expensive, and are also a pretty solid buy.
It's also not a bad thing to mix NOS and modern tubes. Doug Sewell did it with the run of the first 20 DG30 amps (I have one), that included the BRIMARs in V1 and V3, and JJs in the other two positions.
So - again, assuming it's a good tube - NOS preamp tubes aren't such a bad way to blow your dough, and you can add them one at a time to spread the cost out.