There are different schools of thought on whether you ought to go for new pickups or a preamp here.
1.) If you really love the basic amplified sound of your bass.. it's inherent amplified tone.. and all you seek is a way to adjust in some punch when you need it, then that argues for leaving your pickups as they are and going the preamp route.
2.) If you find the basic amplified tone of the bass lacking.. then it's time to look at changing your pickups.
Fullrangebass is right, the J pickup post that's stickied at the top of this forum is an awesome resource.
For my own opinion.. I've recently found "similar to a musicman but not quite as harsh" since putting Nordstrand NJ4SV pickups and an Audere preamp in my bass.
* It's not truly a MusicMan tone, but it's within spitting distance, so I can tell you that it is possible with a J. I'm not saying that you need to go the route I went, I'm just saying that it's possible.
In terms of new pickups.. when I think MusicMan-in-a-J, those
Delano J pickups with the massive single polepieces keep coming to mind.
* High-Z, moderate bass boost, slight low mid cut, slight or no treble boost to taste, favor the bridge pickup by about a 1/4 turn