Some of you may have read my first post, Phase I
Phase I preamp swap complete
where I took the preamp out of my Brice HXB 406 fretless and put it into a Doulgas WOB 826 fretless. The Douglas sounds a good deal better than it did with it's original preamp. Didn't sound fantastic, though. Started thinking that a new bridge pickup (at least) would make it great.
Just got my Bartolini 4.6 preamp harness and installed it into the Brice.
All I can say is
HOLY $HIT!!!!!
This thing sounds absolutely KILLER. Right now, the Brice is everything that I wanted in a fretless bass. The sound is brilliant and ballsy. A 110% improvement. The pickups in it are fine, no need to change them. The bridge pup gives it an incredible growl.
I wired it up for 18 volts just to see what it's like. Never played an 18v bass. I had to tape up the 2nd battery and put it in with the preamp. The 1st batt has a separate compartment. If I feel that it is too much to deal with and too expensive as far as batt replacement, I'll just cut & solder the wires and make it 9v again.
I keep the small gain pot on the preamp at the minimum setting. Through my SVT 2 pro and just the 2x10 (Neo equipped) it sounds tremendous. I'm scared to plug in the 115 in the house.

Things may start to fall and break in the apartment below me.
It came with a plastic stacked knob for the Treb/Bass pot. One day I'll get a stacked knob to match the rest of them. I had to enlarge 2 of the holes in the bass just a bit.
One thing that kind of sucks, but I can get used to it, is that the knobs are now arranged differently. Originally, from right to left, it was Vol, Blend, Treb, Bass; top to bottom when facing the bass.
Now it's Blend, Mid, Vol, Treb/bass
I read somewhere that they use enough slack in the wires to arrange the knobs as needed, but that really isn't the case. The wires are pretty short and tightly strung.
I used the original Brice input jack because the one the harness comes with is sort of cheesy and "cheap" looking. There was also one wire that came detatched from the MID pot; had to resolder it. No biggie.
So now going back to that Douglas bass. I'm wondering if new pickups would not make such a tremendous improvement as a better preamp would. I was thinking about selling the Douglas as is. I actually stuck it in a box all wrapped up and ready to go. I really don't need 2 fretless 6-strings. Like I said, the Brice at this point is all I ever wanted. Maybe I'll put a Bart 4.5 2-band preamp in it and then sell. I'm sure it'll sound killer then. I got a decision to make...
Funny, I just realized that I got a 4.6 A/P preamp, which has an active/passive pull pot (volume). I ordered a standard 4.6 that is just always active...bonus I guess.