There are 2 things you need for a real nasty distorted bass sound:
Tech21 Bass Driver DI Bass Pre Amp Pedal and DI Bo at DV247.COM
Boss ODB3 Bass Overdrive at DV247.COM - has separate controls for the low end and the trebble, also if your running an EQ pedal you can boost the low mids nicely when mixed with this - a must have in my opnion.
Has anyone else got any suggestions - maybe some amplifier suggestions?
Tom