yamaha rbx350 bass pickup upgrades

[sid28]

Hi, I want to upgrade my pickups in my 4 string passive bass guitar. It is a Yamaha RBX350 4 string passive bass. It has 2 P style pickups at the neck...the 2nd starts in the middle of the one closest to the neck...like a p-bass, then it has a J style at the bridge. It has a volume, a tone, and a blend knob. Basically I love the way this bass feels and I want to soup it up. Honestly I'm not exactly sure of what sound I specifically want...but I'd like a setup with as many possibilities as I can get. Definitely a low, defined, powerful clean sound, I'm not really wanting a distortion type pickup...if that helps. The 2 P's are 2 7/8" across and 1 1/8" height. The J is 3 3/4" across and 1 1/4"...height where screws go 5/8" at end. The 4 strings are 2" across with 5/8" between each string. I'd like the easiest switch, but I am "slick" enough to do a more advanced upgrade. I'm also curious about other upgrades for this bass...wiring, knobs, going from passive to active, etc. Any info, thoughts, etc. anyone could provide me on this would be greatly appreciated. Thanks in advance.