Seems like the best "upgrade" for tube amps....

That's good to hear about KT-120s. I have been using my pair for over a year so maybe I have a few more years to go. Mine are run pretty conservatively----500v and 65ma---so that should help.

By the way, I don't see any point in waiting very long before playing music through the amps. I usually let mine warm up a couple minutes (while I am sorting through LPs) but that's it.

I have noticed the same thing with my tube amps, both my flea-powered 46 SE amps and my 50W PP amps. After 2 hours or so, I can hear greater detail and the soundstage has more of a see-through transparency. It's not a night-and-day kind of thing, but the music consistently sounds better a couple hours into a listening session.

I don't know any technical reason for this but it may have something to do with the heating of the transformer cores. A year or two ago I replaced the power transformers in my 46 amps with tranformers rated for higher current, and I notice the "warm up" differences much more now than I did with the old transformers.

Someone once said something along these lines: The sound quality of a given component is 75% due to the circuit design, i.e. the engineering, 75% due to component part selection, and 50% due to build quality.

I like this not just because it's funny but also because it reflects the truth that a given person can focus intently on just one of these aspects and can improve sound quality significantly and then conclude that this is the most important factor. For example, a designer might spend lots of time trying different operating points with his tube circuit, or a modder might spend lots of time trying different capacitors, or a builder could try different layouts and different chassis materials. Each one will find that some variations are better than others and there may be even be a "best." But this experience does not mean that this one approach (circuit design vs. component swapping vs. build quality) is the only or even the best road to good sound. They are all obviously very important, and none of them greatly outweighs the others in my opinion.

From the other posters' comments, it seems Frank Van Alstine is mostly in the circuit design camp, but I am sure that even he would acknowledge that component quality and build quality play some role in the sonics.