Would you buy a tube amp if you were unable to use vintage tubes in it ?

If you bought it new with new tubes, you liked it with new tubes; if it turns out it sounds even better with old tubes, that is just a bonus.  Most of the time, I prefer carefully selected vintage tubes.  But, it can be quite a process auditioning various types and brands.  For the gear I own, my linestage and amp have very limited modern options; if I stick with the correct tube type—348, 349, 310–it is primarily Western Electric and scary expensive (so far, I haven’t had to replace a tube in about 15 years of operation.

I stumbled into NOS GE 6SN7GTB small tubes years ago and haven't looked back. They last REALLY long, use 'em in my Freya and Had Firebottle. Nothing sounds better to me (emphasis on "me"). Power tubes get changed around for fun and are all newish...Gold Lion KT whatever, Sovteks here and there, cool looking huge Chinese rectifiers, and sometimes weird transistors in my Pass XA-25...those things sound like tubes sort of so they get mentioned...sorry.

15 years without replacing a single tube. That's incredible.

Vintage used could be $100 each, but if you wanted to match dozen EL34 tubes..that would be a problem. And, yes, you couldn't be sure in advance of significant improvement. Well, vintage tubes were not created equal.

Actually I would  need two matched sex-tets, but still. . . .

Yeah, Brent from Audio Tubes might be able to deliver it for you if you place a custom order, but the cost..probably at least $150 each for used test NOS, more likely $200 each.