i have a friend selling a Randall RM4 preamp - a v good way into it
The older Egnater M4 that only took single channel modules was exactly the same as the Randall RM4
It lost the loop when it got the dual channel module facilities
The randall modules are good - my faves being the Lynch `Mr scary , and Brahma, Plexi, and `Kirk Hammet KH2
The Salvation modules give you stuff like ENGL, Diezel, VHT, CAE, Framus etc too