Oh puleeze, Mr. Killer Amps I Doth Covet! I think Dave knocks his work out pretty quickly! At least he did with MY amp! You'll get your amp soon and can then add it to your sig!
The fat switch is extremely handy if you want to do "gain shaping" with different guitars. It actually adds gain to the preamp mostly in the low lows and low mids. Because of this, the high end is smoothed a little too. Just mucking with the eq and dialing out bass and adding mids can render most of the mods (and the Marsha) into almost VOX territory: fatter preamp gain and thinner power amp gain; if that makes any sense. It's useful.