I admit it, I'm a fan of NOS tubes. Not all of them, and not in every amp, but when the combination is right...it's hard to beat.
Not all that long ago, tubes were not merely for guitar amps and hi fi nuts. They were an essential technology and they had to perform.
They had to work in (for example) hospital equipment, jet fighter and bomber aircraft (yes, really, and one reason was that they were supposedly less prone to malfunction in the event of a nuclear blast), tanks (all tanks made after the start of WWII have radios), car radios, televisions, scientific equipment, telecommunication, computers, and of course in recording equipment and audio gear of all kinds.
They were, in general, made to a higher standard than most guitar amp tubes are today in order to meet these functional needs, and on equipment that was newer at the time, better maintained, and involving individuals and companies with far more experience in quality tube manufacture. And of course, tubes that were made to the highest standards were marked for military use; they weren't much different from their civilian counterparts, however.
Today, if you've seen pictures of primitive-looking Chinese tube factories, and know about how old some of the Eastern European equipment being used to make tubes there is, you'd have a hard time believing that tubes were ever high-tech. But they were indeed.
I've tried lots of brands of both old and new tubes, and there are audible differences between the brands, as well as the tubes of different eras. There are also differences in how long they last. I won't say whether a sound is better or worse, that's subjective, but in my experience in guitar amps revealing enough that the listener can hear a lot of detail, each brand sounds different.
With lots of amps on the market, and lots of new and old tubes still available, it is possible to use these differences as sonic seasoning, and tailor an amp's tone to a degree.
And yeah, we're not talking about huge differences, some can be subtle. But I'll say that replacing a set of NOS Chinese 12AX7s with a set of NOS Telefunken Smooth Plates in one of my Two-Rocks was a very interesting exercise that changed the character of the amp. And interestingly, I preferred the 80s Chinese tubes in that amp. Though I wound up replacing both sets with some ANOS RCAs that were the best of both worlds. And a set of NOS JAN Philips small bottle 6L6 WGBs really improved things compared to the modern glass the amp shipped with for power tubes.
Why do I bring this up? Well, my DG30 was one of the few that came with NOS Brimar tubes. Brimar was actually an ITT subsidiary and stands for something like British American manufacture. In any case, the tubes were very high quality in their day, and have a unique sound, very round on top, sparkly, and to my mind, beautiful sounding. I'm working on finding more of the right spec.
For me, a big commitment to an amp is when I get some NOS glass for it, and I found a set of Russian mil-spec 80s EL84Ms for the power section, that I'll install at some point (right now the stock modern tubes aren't even broken in, no sense in replacing them yet).
I'm also planning to find some NOS Mullards for the HXDA down the road.
So...thoughts on tubes, folks? Whether you think NOS is great, or a complete waste of money, it's all good, it's just fun to talk about this stuff!