Well here I am going to diverge from the pack a bit. Don't hate, just offering a different position. Tube technology is very old and its true that advances seem to have been on the whole limited for many years. But that said, although most of society has chosen solid state for its obvious advantages, there are certain things that are good about tubes that solid state cannot match or at least only with outlay of serious money. If you listen to a "modern" tube amp, and by that I mean one made in say the 1990s or newer, it is unfair and I would suggest inaccurate to compare these amps to a Model T. These are not 100 year old equipment, but relatively modern designs in many ways, that happen to be powered by tubes (I am oversimplifying to be sure). And there is a clear difference in the sound of even a decent midrange priced solid state amp and a decent tube amp. Now, you many not like the way a tube amp sounds and many people will tell you that it is less "accurate, or precise" whatever those terms are interpreted to mean, and if your goal is to recreate the sound from the mixer as the recording was produced, you might be right. Or you might not. All applications are a series of compromises that often affect the final sound. And if you are playing, say, a Beatles album, the equipment it was recorded and mixed on likely had lots of tube in the signal path. So which is closest to that stated goal? Is that even a goal to which we should aspire? Or is it more like art, or wine, where you know what you like even if the critics disagree? I think I fall somewhere in between. I am not interested in chasing the sterile pure studio sound. I have compared amplifiers and my ears prefer certain ones over others, and since I cannot say what the engineer was trying to attain, my enjoyment is all I care about. (I am a baby boomer, after all!) So to me, tubes DO sound different and in my price range, I prefer that sound. I make no claims to the utter accuracy of that sound, again, whatever that any mean to an individual.
But if I accept that as true, the next question is whether there is a difference between tubes. There certainly is. But is it old tubes or new Russian or Chinese tubes that are best? If your tube is in good order not leaking, putting out the correct amount etc. then there should be few differences, right? Again, I think there are differences. They are subtle, absolutely. But they are there. Just as there are differences in the design of speakers, so there are differences in tubes, since different manufacturers used different practices and materials to produce them and different applications. So it should come as no surprise that there are differences. This is a problem since the only way to tell which you prefer (vs. which is 'best') is to sit and compare hopefully matched sets under similar application. This is really hard because of all the issues of psychology and memory and so forth since it takes time to switch them over, and bias them even in the most efficient, several minutes. If you could get a couple of identical amps and switch back and forth you might get an idea. I believe there is a difference between the newer tubes, some of which are very good, while some are pretty awful sounding TO ME, just as some older tubes can sound wonderful and others awful. So in the end I ended up with matched sets of GE tubes in my Conrad Johnson amps. I sampled about 4 different brands, some new, some vintage. My GEs are from the 1970s so there's none of that WW 2 German tube stuff. I can't afford them and I doubt I could hear an advantage, although I have never tried, since I can't afford them.
As I have noted elsewhere, I use an Audionics Space and Image Composer for nearly all my analogue source processing. I love this thing and have used it since about 1979. But it is a compromise as all things are. It makes some mids and highs sound a bit brittle, in my opinion, so coupling it with my tube amps reduced that effect, to me. Two wrongs make a 'right'? Perhaps. In the end it just is the combination that appeals to me, and as I said, that's all I care about. Sorry to go on so long. And I hope I have not offended anyone. Although it wouldn't be there first time.