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

its not just true of tubes...its true with all high end equipment(especially tube amps)...it simply is not built to be used for years and years..hell, most audiophiles look at'months'like a longterm investment

I heard the wood laid in the canals in venice for at least a century before it was fileted to make the great instruments of yore. For tubes, I'll bet they simply cull fewer of the slugs from each run than they used to. So your chances of getting junk are higher. I'm with Grogan, sell 'em crap they have to replace sooner.

I do think some poeple are finding out that you can increase the performance/and longevity of a modern day tube by having it cryo treated.This will bring the so called cheap tube much closer to the NOS tube. Companies like Tube world sell cry'od tube and people are buying like crazy. Some tubes seem to do better with this treament than others. I have bought cables from a company call Ridge Street Audio that will send out your tubes ( at a fee of coarse ) to have them cryo treated and send them to you. I am using Svetlana EL34 and 12ax7s that I will eventually do just that.

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.