This sounds like a really cool mod. Would like to know more.
Here's my bass:
Mine has an additional Dimarzio Will Power pickup in the neck spot and an extra volume and switch.
I've found that I rarely use the preamp in my L2000 Tribute, so I removed it.
Here's the wiring diagram for the L2000/L2500 series basses.
Trace the circuit from the pickup selector switch and you'll see that it goes pickup selector switch > treble pot > bass pot > volume > preamp/active switch > output. There are two white wires connected to the middle volume pot lug. Cut the one going from the volume to the preamp right at the volume pot. Cut the one connected to the passive/active switch about two inches from the switch. Cut the white wire coming from the preamp to the back of the bass pot right at the pot. Connect the remaining white wire from the volume pot to the white wire connected to the output jack, and you have now made the bass passive only and freed up a switch hole.
If you want to do the separate series/single/parallel switches then you'll need two DPDT on/on/on switches (I'd probably buy three in case you screw one up). Here is a diagram for separate series/inner single/parallel switches:
I used a push/pull volume pot for that middle switch wired so that the capacitors are on the opposite end from the pot part.
Once you have the preamp cut out of the circuit and the first part of the wiring connected in passive you're just redoing the switches and volume pot and then connecting them to the pickup selector switch. I'd cut the white wires connecting the pickup selector switch to the stock series/parallel switch right at the switch and use them to connect to the new switches.
Sorry if that seems really complicated! I'd be happy to walk you through it if it's something you'd want to do.
