Why aren't new vacuum tubes as good as old ones?

I think you should avoid the 6SN7s from that manufacturer as clearly something's wrong...but preamp tubes or driver tubes generally should last a very long time. Also, 250 bucks is some large cash for 4 driver or preamp tubes...sort of my lifetime preamp tube cost, although I can see (at thetubestore anyway) 6SN7s seem pricier than some other preamp types.

You haven't found the right modern tubes then? Emission Labs and some of the special Shuguang (nature sound range) are superior to the best vintage tubes IMHO. But I guess you are stuck if they don't make the tube you need?

I think the vintage tube obsession, unless you cannot find a good modern version, is bogus.

Nearly all the vintage tubes I have had over the years sounded Dark, or metallic, grainy or probably too fat or overly saturated. Try the mesh plates from EML. They are wonderful (and expensive!) but the nature sound are different sounding but as good and probably a lot cheaper than the EML

There is also a Japanese company - Takatsuki - that makes a few tubes. . . notable the 300b. Very expensive, but also very good.

New production tubes that I can vouch for
1) EML.
2) Psvane, particularly their top level W.E. Replica series.
3) Shuguang, their treasure series.
4) Takatsuki,
These can compete with the best tubes of any era in my opinion.
There are some very good NOS tubes to be certain but there are lemons in that niche as well.
Charles,