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

Good question, I believe it's because 'they don't build 'em like they used to' philosophy. In business today, it's all about the bottom line, getting the fewest amount of people to do the most work possible. In the older days, the quality was priority. Now it's how to make it cheaper, so the bottom line looks better. Profit margins are the drivers today, it wasn't like that BR (Before Reaganomics). Business' are run like there is no tomorrow, make the most you can TODAY.

While there is some validity to Jmcgrogan's point, remember that there always has been a profit motive in making tubes even back in the golden age of tubes, just not as emphasized as it is now. In fact, in the right circuit some modern tubes can sound as good as NOS, although I don't think they're as well made so I would question their longevity. I think the biggest differences are in the quality of the materials used to manufacture the tubes and the skill/experience of the persons building them; there seemed to be better quality control of these aspects in the past, from what I've been told, and this does come back to cutting corners to improve profitability, of course. This explanation would work somewhat with your violin analogy, as there are some fine craftspersons who care greatly about their work building violins today which sound quite good, but can't duplicate the sound of the great Strads and Del Gesus of the past (yet--let's see what they sound like in a few hundred years, when the wood has aged). There are all sorts of theories as to why, but I subscribe to the one that it's the wood that was used, not just the type but also the purported fact (which I've never been able to verify, so don't hold me to this one) that some of the wood came from old churches and thus was hundreds of years old when it was made into the violins. Don't know if the latter part is true, but sure fits my theory nicely!

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.