Ok, if he's serious about getting into building, tell him to pickup a russian big muff pi, then put a blend knob in to let him blend between his clean signal and the dirty one. He can also ajust the tone stack to work better with a bass. The biggest reason bass distortion sounds like crap is because it clips the soundwave and gets rid of the smooth thunp that makes bass tone. So by mixing dry with distortion he'll get the best of both worlds. This will work with any distortion or other bass effect. He may just want to build a wet/dry loop so he can mix all of his effects this way.