greenboy knows a LOT about this. Seek him out if you dare.
In a nutshell, though, it's difficult to use a bass as a MIDI controller for the same reason it's difficult to get a really killer synth bass pedal: it takes too long to sample the notes because of their low frequency.
If you go this route I'd suggest getting a piccolo bass and adding a MIDI device that samples each string separately. That should be your best bet.
Unless greenboy says otherwise, of course.
