I'm not completely sure, but to my ears I can hear some Les Claypool-like tone in his sound, and it might have to do with the electronics in the bass itself.
If this is a band that you dig, check out their website. A lot of players will reveal their arsenal in some sort of equipment list, and that might help point you in the right direction.
And sometimes the primary factor in getting a certain tone is more closely related to the guy's playing technique, rather than the equipment itself. You could conceivably match every single component in the guys rig, from bass to speaker cabinet, and it might still not get that same tone.
Just to toss this out there, I remember a story from way back when about Van Halen when they opened for Ted Nugent during their first tour. Ted was so blown away by Eddie's tone, the he asked Eddie if he could plug into his pedalboard during soundcheck to try it out. The story goes that, even though Ted was now running through Eddie's rig, it still sounded like Ted.