If we can figure out what/where the bass cut is in the Grunge pedal, we should be able to make it into a beast of a bass distortion pedal.
I seem to recall cutting out/removing the C26 capacitor makes the Grunge less noisy and better sounding.
I also have a Boss Metal Zone that I want to mod.
I love my DOD Boneshaker for bass. It seems to be better suited for baritone guitar and bass, than for regular guitar.
That's another dirt pedal you could get for bass distortion, and it has a parametric EQ.