The mod is very similar for the Caliber .50 and the Studio .22.
Basically your adding a 1 Meg pot and changing the 470k resistor to 1 Meg after LDR 2. This gives you and independent gain control for the lead channel leaving your original gain control for the clean channel.
Looking for the schematic now. PM me if you want it.
It was a factory designed mod so any Mesa authorised tech should be able to do it.
Cheers!