JJLipton said: variable clipping? That sounds really interesting.
It was a special circuit patented by the Samamp guy. The reason they're not well known is because they are hand made by one guy in Birmingham Alabama.
V.A.C., or Variable Amplitude Clipping is used to redue the output power of the amplifier by sending the signal through a series of lightbulbs in the back of the amp to cause the power to "sag" and lower the clip level on the amp to gain the same kind of distortion achevied by turning the tube amp up full blast. Basically, it does something a little like what a Variac does on an amp, it lowers the availible power to the power tubes, causing the amp to break up and distort earlier.
From my experience, the amp I tried was the green combo pictured above, which had 5 channels if I remember right, and we had it set at 30 or 15 watts, though I'm not TOTALLY sure which. It had a Fender-ish channel, another channel to sound like an AC30, another for a Marshall Bluesbreaker type sound, and the final one was a very Van-Halenish Marshall Plexi Sound, the first and the last being the two I really liked, especially if it can get that blasted Plexi sound out of a stock mustang, that was unreal. I believe the benifit of the circuit is that, unlike power brakes and some other things, the sound is much warmer and mellow than that of say, a Marshall JCM with a power brake.
http://www.samamp.com/page2.html - Thats where they go into it more in depth.