One of G&L fortes is their MFD pickups. Honestly, there are a lot of replacement choices out there. Perhaps your best option is selling your L2500 and getting either a M2500 (if you like everything else about G&Ls), or look at other brands of basses entirely.
G&L are great basses, and most folks that own them love the factory pickups. If you like the bass, my suggestion would be to start somewhere else to improve the tone rather than replacing the MFDs, which are great sounding pickups.
Review exactly how the knobs and switches work, and get a clear understanding of what each setting does.
Quick list:
- Replace strings and battery, and a professional set-up
- Zero out your amp tone controls, and reset gain structure
- disconnect any effects between your bass and your amp
- You may want to lower your pickups a touch
One final point, in the event you pull the factory pickups out, you will probably need to install completely new electronics and wiring harness as well. Also plan on never selling the bass, because no one will want a G&L with replacement electronics and pickups.