Tube theremins have a tendency to "pull" each others oscillators on the low notes adding a certain color to the tones that only tube theremins can produce naturally. This is a side-effect of inadequate decoupling between the stages combined with the fact that tubes draw a lot more current and have much higher dv/dt's than transistor theremins. This effect only really happens at lower frequencies where the two oscillators are close enough in frequency to actually pull each other, once they get a certain distance (frequency-wise) away they tend to work more independently. What this does is make the low notes nice and fat and the high notes the more pure sinusoidal shape that theremins produce (i.e. ballsy low notes and singing high notes). The other thing that tubes beat transistors in is the distortion that the tube stages produce naturally, something that a few SS theremins attempt to emulate but never nail.
With that said I love my Etherwave theremin (well Big Briar, but same difference) and would never trade it. My dad built it when I was a young'n and I quickly took it over and learned to play it. Very fun instrument and not always completely useless.
Also the PAiA theremin is a lot of fun and has some unique features. The built in CV outputs are great for controlling a synth with the theremin and you can mix in a square wave again to make this thing very usable with a synth.
There are quite a few schematics online. I think my first build is going to be the Moog tube design that he posted in his article. Uses common tubes and isn't overly complex..
Also here is one of my favorite (ab)uses of theremin. It's a crappy cable access program which features a theremin, a chapman stick and some weird midi drum thingy. The Chapman stick player is terrible and the rhythm player is goofy as all get out, but the theremin player makes some intense faces! Seriously, watch it until the end, it's worth it.
http://www.youtube.com/watch?v=EU41sOFvQWYCliff Schecht - Circuit P.I.
