I just had the parallel / series switching done on my 5 string SX jazz bass.. WOW what a difference in tone.. I had them replace ALL of the pots with new CTS pots, orange drop cap, its a push pull as described in the diagram below.
Also had a pro setup done, with crowning and leveling, new pure graphite nut and new high mass bridge installed..
The bass sounds so much better, clarity has been enhanced, muddiness is gone.. Solid thump from the B string now.. If you guys and gals out there that can't afford and active circuit or don't like an active bass and want a mod done that is practical and toneful.. I would recommend this mod as one of the best for a jazz bass or similar dual single coil setup bass.
Here is the diagram:
