The Sonic T-amp is in the category of "Class D" switching amplifiers. Most are fairly low output, but with high efficency speakers (>90dB) will work just fine. The principle of low-power tube amps, as Gonk noted, has been around for a while -- case in point single ended triode (SET) designs.

The Sonic T uses a tripath chip. There is also the ICEPower platform. Though both seem like toys compared to linear designs like those produced by Outlaw, etc., the technology is very promising and has been adopted by high-end companies like Rowland Research, PS Audio, Red Dragon Audio, Bel Canto and NuForce. Unlike the Sonic T, these amps put out as much as 1,000 watts per channel in a chassis that can darn near fit in the palm of your hands, produces virtually no heat, and draws very little AC current. Unlike the Sonic T however, these amps can run upwards of $6,500.

If this kind of geeky thing interests you, buy a copy of the most recent issue of the Absolute Sound. They have a special feature on Class D designs.