Why do old tubes sound better than current tubes?

Nostalgia?

Huuuhh???

you know. that old time sound. tubey.

Does Flo Rida or the Black Eyed Peas sound better with NOS 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).