| as an analogue synth fetishist, i can tell you why it's tough to get a good synthetic sound out of a bass - a lack of filter tracking.
to get a good bass synth sound, you need to use a lowpass filter with some resonance, but most importantly the filter has to track the pitch. with a regular lowpass filter on a bass, the filter cutoff stays at the same freq no matter what note you play, so high notes have the same filter cutoff as low notes.
On synths, it's an easy enough thing to accomplish - pretty much ALL synths let you do this, but it's more difficult with a stompbox.
you would need some sort of pitch tracking device controlling a lowpass filter. the moogerfooger lowpass filter would be a good choice , because it allows to you control the filter tracking from an external source....what you need then is some device that will output a CV (control voltage) signal based on the pitch of the bass notes. the only thing here I can think of would be an octave pedal, but I have never heard of one with a cv output (or any output for that matter) of the pitch as a cv (does the POG maybe?)
there are a few synthesizers than can detect the pitch of an external audio signal & output a CV (korg MS20 is a good example), but of course getting a synth in the picture sort of defeats the whole purpose here....
i think if you could find such a device, what I would say would nail a great synth sound would be:
bass -> the mystery box that makes a pitch CV -> controlling the lowpass filter.
you could further approximate a synth by adding dynamic filtering (which I believe the moogerfooger already does), as this is something that is often added to synth bass.
I've got a minimoog here & I bet you could get pretty close to a minimoog bass sound using the setup i just described.
anyone know of a stompbox that outputs pitch info...?? | 
