I bought a Kawai F2b which had a defective bridge pickup, which had a lower output.
I really should have left it alone - tbh I could have lived with the lower output, especially as I'm predominantly playing 5ers now.
Changing the pickups turned out to be a nightmare - I ordered a set of custom pickups which really were characterless and had to go back.
Replaced these with a set of active Wilkinsons which are "fine" but not as punchy as the originals.
The whole process took far longer than it should have and I'm not convinced that I've upgraded the electronics - if I was playing 4s I'd probably be thinking about a good quality preamp but since I'm not I'll leave it for the next owner to do if I sell what is an amazing bass.
Images below are of the original pickups.