Bluesbreaker Mods:

I don't have a Marshall Bluesbreaker, but I built one from Gus's schematic. The Bluesbreaker is designed to make your cheapo tranny practice amp sound like the Marshall Bluesbreaker combo. It gets about halfway there, and can be had for about $50US. It can go from clean to bluesy distortion, and can be used to overdrive a tube amp. It doesn't have the extreme midrange boost of the Tube Screamer, and sounds more amp-like. If I had the original pedal I would leave it be, but if you want to build one yourself you might want to fool around a bit.

Proposed "enhancements":

I increased the bass response a little and added a switch to give a bass and treble boost at the same time. This switch gives you a flatter frequency response for when you get bored with the Marshall vibe. At the moment I've replaced the silicon diodes with the Aron Nelson "2 IRF520s as MOSFET diodes with a 1N34a diode attached to the source pin" diodes, and I decided that this sounds more "amp-like".

(Update: The Bluesbreaker II has a new tone control, how interesting)