Why do old tubes sound better than current tubes?

Certainly, particular designs, system synergy and personal taste could favor any one particular tube over another. For small signal tubes, I have generally found that older production tubes sound better, and also last much longer.

The big problem with older tubes is that one hardly finds truly new, or barely used, old tubes on the market these days. I've bought tubes that are claimed to be NOS from supposedly reputable sellers that test pretty mediocre on a good, modern tube tester (Amplitrex). Also, there is such a wide variety of voicing of older tubes that it is easy to come up with simply wrong tubes for a particular application.

In everything that I've heard, for my taste anyway, I've been able to find alternatives to the manufacture chosen tubes that I prefer. Almost always that alternative has been older varieties for small signal tubes. But, then again there are a LOT more older choices.

I own modern 2a3 power tubes that do sound VERY good (EML meshplates and solid plates) so I can say that there is good current production of those tubes. A friend has AVVT 300bs that are quite good and distinctive (not as mushy on the bottom as most 300bs, new or old).

Tube are often represented as NOS that are probably old pulls, and often include testing results that turn out to be either false or based on poorly calibrated testers. Most tube testers do not subject the tubes to full power tests (the Amplitrex does do this) or give actual numbers for transconductance and other measurements, so it is hard to get meaningful readings in terms of how much life is left in the tubes.

I don't know to what extent older tubes decline without use, but, I have some old tubes that DO test very strong on my Amplitrex so I know that at least some tubes do not deteriorate with age.

As to whether NOS tubes are overpriced, that really is a value judgment. It is entirely up to each person to determine whether it is worth the cost to conduct the search for better tubes. In every instance where I, or my friends, have tried different tubes, we have found alternatives to what the manufacturer supplied that sounded better. That was the case with my amplifiers, and my amplifiers came from the manufacturer with old tubes (RCA bi-plate 2a3s, red-base 5692s). In my phonostage, I really like the result of using old Telefunken ECC803S tubes. I know that they are now unbelievably expensive; if anyone knows of alternatives that sound like that tube and cost less, please provide such information.

Manufacturers of tube gear almost always use current production tubes because availability and consistency are of prime importance. Since alternative voicing is really a matter of system matching and personal taste, it makes less sense to use much more expensive older tubes when the result (matching customer's needs) is entirely unpredictable. Thus, in almost every case, their is at least THE OPPORTUNITY to "improve" the result by selecting alternatives, whether other brands of new tubes or older tubes.