While I'm blathering about vacuum tubes, I'll also mention that I've been playing with a project to produce guitar amp type distortion without huge volume levels. My idea was to take a cheap, common radio tube (6AU6) and use it as the output tube. This should produce around 1 watt of output. I found a little reverb coil transformer would make an acceptable output transformer. Then I decided to add a beefier tube (EL84) and a beefier transformer, and switching electronics. The thing is that guitar amp distortion comes from the interaction of the output tube(s), the output transformer, and the speaker (and its enclosure). This is actually some very complicated stuff, with each part affecting the others in nonlinear ways. So I figured I'd arrange it so the player could isolate individual overloads to see how they sounded. So with the big output tube and the little transformer, you'll get mostly transformer distortion. With the little tube and the big transformer, mostly tube distortion. Playing with speakers affords another avenue for experimentation. However, it's been multiply delayed (would you believe I'm still moving house?), and remains scribbles on paper and some parts in boxes.
However, another group has been working on open-source home-built stuff, and they came up with their own little 1 watt amp, called the Firefly. It uses a pair of common 12AX7 tubes for the input, tone, and "boost" stages, and a 12AU7 as the output tube. There's even a printed circuit board available, so building one should be easy. The parts list shows where all the parts can be obtained.
To continue the guitar theme (hi maugorn), another group is selling stuff to make your own stomp boxes. Pretty solid looking designs, aimed at replicating famous pedals from the past, affordably. And for people who like to customize their gear, you don't have to feel guilty about modding these repros. They even sell complete kits.
If people out there are interested in this sort of thing, but don't feel like assembling them, I'll just point out that I enjoy it, and I'm happy to assemble kits for people (time permitting).