NOS tubes, why can't modern tubes sound better?

What I once heard is that NOS tubes were typically "vacuumed" much more than the current production which are vacuumed so-so that it just qualifies the "electronic specification" standard. I don't know how exactly that theory is connected to the diference in sound. Maybe the more vacuum and therefore the thicker glass and thus more sustainability to the vibration? As usual, I may be dead wrong.

A semi-contrarian view. On the whole, I've had better luck with tube LONGEVITY using new tubes (where they are available) vs NOS tubes. And in some cases, e.g., 845 output tubes from China @$40 vs their NOS counterparts at God-knows-what prices -- you really have to be crazed or a commited audiophile (same thing) to go with NOS. And in terms of sound quality it's not cut and dried either. New Svetlana 6550s can hold their own against NOS GEs and then some. The best EL-34s I've heard (and I've not hear 'em all, obviously) are the Valve Art EL-34Bs from China. On the other hand, it's hard to beat an original Telefunken 12AX7. My only point is that there are really no absolutes here. IMO.

There is an excellent article in the current issue of Vacuum Tube Valley on this subject. You should check it out.

What I heard is because some material can't be used today. And we love our world :)

A little off topic but another analogy...

Back in the day, maybe still, Kawai reverse engineered Yamaha pianos - used the same materials. But bless them, they just could not reproduce the Yamaha sound.

Craftmanship I guess...