Mesa/Boogie amps offer a wide range of controls for shaping the tone and feel of the amp. Among these are several that affect the AC power of the amp. Here’s what those switches do in their various settings:
Variac Power, Spongy, Tweed — limits the incoming voltage to around 95 volts (depending on what is coming out of the wall outlet). This creates what Eddie Van Halen referred to as a “brown” sound, with reduced midrange; looser, softer feel; and a shift in where the higher harmonics occur. Power also drops, and headroom may be reduced. These settings can increase the life of your tubes.