I like Flat Boosters into Marshalls, like the Radial PB-1 or the MXR Microamp or the MXR CAE Driver.
Treble Boosters are too ice picky into Marshalls IMO - well - I have to say I am talking JCM800 types. Actually I think you might wanna get a treble booster for yours though. What about a BOR? Great Overdrive AND awesome quality booster in one?
in theory, the treble booster ought to be great in a superbass.
i do agree about treble boosters and jcm800 type amps not really working, however some treble boosters have a tone control to tame the high end. i'm using such into a jcm800 type amp.
i have a thundertomate phil hilbourne and i can recommend it, though it is pricey and not really working with high gain humbuckers. massive sustain and great definition with that pedal. crunchy too!
i like the od-1 tone hugely, but it does take out bass, so if you are worried about that the crunchbox would be a good affordable choice - at least from the pedals i have experience with.