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!