Thanks all, a couple follow-ups:
Joe'sBass: you found the OBP-1 to be somewhat more organic and natural compared to the Sadowsky... were they installed in similar basses by any chance? Just curious to know what types of pickups + woods each of these Jazzes had.
I've also read that Marcus uses a Bart pre-amp that Roger installed, and agree that the bass (alder body, maple board) will sound bright... that's what I like from this Jazz!

I'll leave intact the passive tone control, as well as the series-parallel wiring I currently have in there, so this will allow me to tone down the high-end when necessary.
My outboard John Suhr preamp also yields a noticeable boost in highs when I engage it, even if the controls are rolled off. Just lowering the impedance of the output reduces the high-end loss caused by the instrument cable.
stingrayguy: I hear about the 18 dB of boost... that's a huge amount. Even 12 dB (which is what I believe the Sadowsky, and my outboard Suhr, produce) is a boatload, and I typically don't boost either treble of bass that much. So that might obviate one of the benefits of the 18-volt system in my case... which would mean less routing!