I like what Jaquo said:
But I'll actually say it. The preamps used in his REAL basses, the ones he uses on the road, particularly the 76 Jazz that started it all, use Bartolini preamps. The first one was installed by Rodger Sadowsky. No, its not a Sadowsky preamp, and no, he didn't modify the preamp. Its just a stock Bart 2 Band preamp.
His Artist series basses that we buy have a Fender preamp. Perhaps it is modeled after the Bart, or maybe its just a stock Fender active preamp. That I can't say for sure.