Something like this have probably been done before... Yes, Fuzzdog added a 3-band Marshall style tonestack to a Rat a while ago. Oh, and there is Idiotbox Blowerbox, a Rat tuned for bass with less gain with a James tonestack....
This one is quite similar to the latter but has more gain on tap and are tuned for guitar. The James tonestack have Matamp/Orange values but can be tweaked witch Duncan Tonestack calculator. Oh, and there is a diode switch for assymetric SI/MOSFET clipping and symmetrical LED clipping.
