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.
NOS tubes, why can't modern tubes sound better?
Perhaps this is a useless thread, but why can't currently manufactured tubes at least equal the sound quality of NOS? With the advances in science, why is it that a modern factory cannot exceed the sound quality of the past? Perhaps for the same unexplained reasons that modern violin manufacturers cannot attain the 'Strats'.