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.
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'.