I use a Seymour Duncan SPB-1 Vintage for the P-Bass pickup on the neck and a Seymour Duncan STK-J2 Hot Stack J pickup on the bridge.
They sound terrific. But it's also worth considering other upgrades in tandem. My own include using a BadAss II bridge (increased the sustain) and Hipshot Ultralight tuning machines (eliminated neck dive as they are much lighter than the stock tuners). I also use Status half wound strings, but these are strictly a personal preference.
Is the Badass II a direct fit or is there some fitting required?