My very first G&L bass was a Tribute L2500 that I bought used. The original owner had replaced the preamp with a Seymour Duncan preamp which he said improved the sound remarkably. That was a lie. It sounded dreadful. Not that the SD preamp was bad, it just wasn't made for this bass. I even called Seymour Duncan and asked them about it, and they told me they simply did not make a preamp that would work for a G&L L2x00. To me, the preamp in an L2x00 works and sounds great with that bass. And if you bypass it to passive, the tone controls still work...and the series/parallel switch still works...and the pickup selector switch still works. I'm not a guitar or bass wiring expert, though I'm knowledgeable about some of it and have done a lot. And I don't know anything about Bartolini preamps. I have used a number of SD preamps and gotten good results, but this L2500 I bought was a mess. If you rewire an L2x00 for another preamp, you'd best be an expert at it. If you're buying an L2x00 that's been rewired, I recommend checking to see if whomever did it is an expert. This is because incorporating everything an L2x00 is capable of into another preamp's system can get real tricky, though not impossible.