Well, I think the only way to actually figure it out is to buy one and tear it up. The guys at London Power are not giving up the scoop, and the Reeves people are bound by a licensing agreement not to. I think it's great that they don't tell - not a lot of "black magic" left in tube amps anymore. I'm also a huge fan of money, and respect their right to to invent and market a product for financial gain. I hope a ton of players buy their little black box before some schmuck posts their schematic on the web.
Anyway... I think you're right Isaac, sort of Variac-like, but infinitely more user friendly and versatile. I used a Variac on a Marshall Major that I played through a couple of Goodman 15's (so I could get that raunchy, yet way cool John Wetton sound without drowning out the guitarists) and it was cool, but much more cumbersome than just a knob on my amp head. I've used the Reeves product frequently, and the versatility is astounding. They are a little pricey, but a good value for the quality and workmanship. I would pony up and buy one if I played more guitar. Using the gain and power scale knobs you can go from squeeky clean to screaming tube breakup and every level in between. At ANY volume! And not just 12AX7 master volume breakup, or colorized attenuator sound, but pure power tube wail. I plan to get one of the kits for my Sceptre in hope of making a vintage Sunn "Super Amp" without killing its investment value by chopping it up. I'll post the results when I finally get around to actually doing it.