Beautiful bass! As
@Riff Ranger mentioned
I did a mod project on my 1999 SR800 - same pickups and preamp as yours came stock with this one, and I didn't care for the sound either.
The pickups and preamp here are pretty hard to separate since the battery powers both the preamp and pickups. Maybe if you're clever you can figure out how to keep one and replace the other, I am not that clever though. I ended up deciding that I don't really need an active bass so I went with a fully passive system, VVT control. But since I had an extra hole I decided to add a series/parallel toggle switch as well.
If you're familiar with soldering up pickups it shouldn't be too hard, the challenge I had with mine - and just looking at yours I am guessing it will be a similar challenge - was finding pots that were long enough to go through the top of the bass but also narrow enough to fit in the holes they drilled. A standard 3/8" pot is too thick.
These are what I used. You might also be able to re-use the pots already in there if you carefully desolder the wires.
As for pickups I went with a Wilde
P-46 and
J-45 set - they're priced affordably but very well made and designed, and I think they sound great with this setup.
You could still add a preamp to this, though I can't offer any advice there as that's not what I ended up looking into.
Good luck!