The best parts for making tube amps exist now, not 50-80 years ago. There are some pretty cool designs out there although its all too common for newbies to rehash 1950s designs.
I don't subscribe to the idea that older transformers were better. I think that might have been true in the 1980s but not any more. There are too many artisan transformer manufacturers that fly in the face of that.
(I bought a set of Edcor transformers for a little 5 Watt amp project I was working on and they performed great- full power bandwidth past 100KHz. Not proof of anything really but you can get good transformers without having to work too hard at it.)
Of course OTLs allow you to get around that problem. When we came on the scene nearly 50 years ago we had the distinction of making the world's first reliable OTLs. But we were actually pushing balanced operation more than we were OTLs- and so our OTLs were fully differential and balanced.
Balanced operation offers a lot of advantages if the circuitry is fully differential. This is because the distortion signature is so much more benign than you get with single-ended circuits or single-ended combined with push-pull like you see in something like the Dynaco ST70. This is simply because of the kind of non-linearities present in such amps; mathematically, single-ended has a quadratic non-linearity, fully differential amps have a cubic non-linearity (so harmonics are falling off at a faster rate as the order of the harmonic is increased) and traditional push-pull amps have both (resulting in a prominent 5th harmonic, which is why I think most SET lovers don't like push-pull). Balanced allows for lower noise and greater immunity to power supply issues, as well as the ability to reject noise at its input.
There are brilliant amps like the Berning zero-hysteresis design (so-called 'ZOTL', which is an acronym with no meaning; that was something made up by the late Harvey Rosenburg) however such amps are actually hybrid as they rely heavily on a solid state switching circuit running at about 250KHz.
Overall its pretty easy to demonstrate that the best tube amps are made right now. But that's not to say that older amps can't compete if properly refurbished- since there are better parts now, you can easily get older amps to outperform their new specs.
