Why do old tubes sound better than current tubes?

Tvad, You are right. I did read too much into it I think.

New production (or recent productions) I have found a place for:

SED 6550, SED EL34, SED 6L6GC, and SED KT88's (I really like SED's)

Re-Issue Tung-sol 6550 (in one amp)

JJ and NOS Tesla 6922's and JJ and NOS Tesla 12AX & 12AU7's. I like the NOS tesla's better, but not that much and they are hard to get and can be spendy.

EH6922's (one application so far - not universal by a mile).

Re-Issue Mullard 12AX7's (one application so far)

EI 12AX7E (long grey plate) - my go to NP 12ax7 as a starting point/reference. I really like this particular EI though I guess it is out of production. I've had no QC problems with this one either.
EI 6DJ8 but it is not as quiet or rugged as I like. Again it is no longer in production.
EI 6CG7 - the only new one in production. So far so good but I'm just starting an inquiry into this tube type for a specific amp but so far so good.

And, FWIW, I've got a few others that are still trying to find a good home. Curiosity may have killed the cat - so far I'm only limping! :-)

A question for the assembled experts here with respect to small signal dual triodes.

I'm wondering how significant a variable in all of this gain or gm mismatches between the two triode sections within a tube might be, and if that might account for some of the divergence of experiences with these kinds of tubes.

I recently had occasion to test about a half-dozen absolutely NOS 1950's RCA 5963's (12AU7 equivalents) on my Hickok 800A tester. All of the tubes measured well and matched each other just about perfectly. However, all of them had close to a 20% difference in gm between the two triode sections.

As someone with minimal tube-rolling experience, I'd be interested in comments on how common that kind of difference (or worse) tends to be on both vintage and current production tubes, and on how significant it might be for each of the kinds of circuit functions dual triodes are used for.

Re Grant's question, fwiw I'll mention that in my Paxthon VTA-160 I've just replaced a pair of 1960's vintage used Telefunken 12AX7's, which had measured well until one of them recently developed a problem, and a pair of the NOS 5963's, with matched and balanced pairs of Genalex Gold Lion reissue ECC82's and ECC83's. My early impressions of the new tubes are very favorable, with improved clarity on high volume peaks, and lower noise. (The power tubes I've been using all along are a matched octet of SED "Winged C" EL34's).

Thanks!

-- Al

tung sol 12ax7 gold pins, very musical, sovtec lps 12x7 , very open great bass , but maybe a little less body than the tung sol.