Hey StarchMan, the best way I can describe the overall results of the preamp swap is - it was like removing a blanket from my cabinet (if that means anything). The tone is cleaner and I think the slap tone is much better. It sounds like the pickups do with the old preamp bypassed but with a stronger unfiltered signal. I find the bass is more flexible, centered just right. The find the highs can be quite powerful when boosted but I've been keeping the treble turned all the way down for now. I put new strings on so that probably has more to do with that. Another added benefit is the headroom due to the 18 volt setup, I notice my amp doesn't clip nearly as much. Also, I had it wired so that I can still bypass the preamp with the volume knob if ever desired. Bottom line, it still sounds like a Warwick but better (to my ears at least). I think the pickups play a more integral role in the sound. I swapped out the MECs in my old Corvette Standard for EMGs and although the bass still sounded great, I found that it did loose some of the Warwick sound.
Now the one thing I can see as a negative is players with a hard attack may find the OBP-1 preamp a bit too much since you can't cut frequencies. I play with a light touch and like low action so it's never an issue with me. Maybe a proper setup to your playing style is what's call for here. I know Aguilar has the 3 band EQ available and you can cut frequencies with that preamp but I'd rather not tweek too much. Right now, I keep the treble flat, adjust the bass for the room, and leave it.
I would suggest trying to find an outboard version of the Aguilar pre to try out somewhere. Bring your bass, bypass the MEC preamp and run through the outboard preamp. At least you'll get an idea of the overall sound. Sorry that was probably way more than you needed.