With tubes, it's anybody's guess which works in which amp. Usually IMO no sense in buying expensive NOS tubes by the truckload but I always do keep one or two very nice NOS preamp tubes around.
My take is this, based on owning and pretty extensively tweaking well over a dozen different tube amps over two decades:
- Usually the very first tube is the only one that will make a huge impact in the preamp section. Some tubes just sound shrill there, some much more saturated, some warm. But I find finding a suitable tube in V1 can make or break an amp in some cases. Probably even more so depends on what you drive it with - my EMG's at least work way better into a good NOS tube (warm crunch) than a mil-spec Sovtek (shrill buzz).
- In some amps that thrive on the power section for their sound, power tubes can have a clear difference. Depends totally on the amp&how hard you drive those tubes, on most of mine I never found there to be that much of difference, but for instance in the 18W they do change the sound pretty clearly. Which brings me to the next point:
- The simpler and less PCB and chips the amp has, the more the tube change can impact the sound.
- Mil-spec tubes usually tend to sound tighter and colder than 'normal' ones, so they are mostly power-amp stuff IMO, or good for tight drives&modern sounds.
- You never know what works for what amp till you try. Some work the best with the cheapest Sovteks possible, or mil-specs, some amps come truly alive or get rid of some annoying buzz only when there's a very good V1 preamp tube. And, of course, it all comes down to taste. But to sum it up - I don't really think it makes sense to seek those great tubes ad nauseam - just test the first one if it makes a lot of difference and that'll be 95% of what you can do to it with tube changes in most cases.
That's not to say having an old Twin filled with superb Mullard NOS glass wouldn't make for superb sounds, just that in real world situations it's the V1 that seems like the best bet to try. If that won't change the sound significantly, changes are no other change will change it even that much.