First things first, here's the result, with the OBP-3 fitted.
So, left to right - we have:
• Stacked volumes for the pickups
• Push/Pull switched mid
• Stacked bass/treble
• (working!) active/passive switch
• (currently not working) LED for actives on
• New slot-in bridge, replacing the old string-through one.
The basic idea was that the bass wouldn't need any new holes drilled in it for pots: I wanted the OBP-3 as flexible as possible as well. Definite requirement to stack volumes!
As detailed before, the existing volumes (the pickups are passive J-type) were 500KOhm - as luck has it, this is the same as the stacked pots for Jazz basses.
However: the pots are stumpy, being designed for mounting on a J-Bass control bridge. This meant I needed to take a couple of mm off the inside of the control cavity, but I realised that careful routing of a rectangle using a DREMMEL would hold the pot in place as well, by the metal bracket on the top. Problem solved, it's solid with no washers required and without any risk to the top of the bass.
Everything else was reasonably simple - the wiring for a stacked pot with active/passive switch isn't in the Aguilar documentation though, so a melding of 2 diagrams was required, which acknowledges that the pickup signal output from the pots needed to be wired into into the switch where the line that would have been the volume control went.
Found that out using Basschat UK - they rock, btw!
The bridge was an easy mount: slightly wider than the existing one, so careful measurements have me good pencil lines to get the positioning right: luckily the new bridge has a 5 hole arrangement where the old one was 4, so new holes exactly in between the old ones. New bridge has good tone, the low B is spectacular, a great improvement and tight as you like: I'd recommend the upgrade to anyone. It was a Göldo from Thomann, available from https://www.thomann.de/gb/goeldo_bass_bruecke_5_chrome_hw55b.htm and only €27!!
The knobs - however - hard work. The stacked pot needed a set from Allparts: £12! This is for 8mm/4mm and fits the hideous imperial pot size. The mid is a pot from my Tobias: the stacked bass/treble needed an "alpha" fit pot - now, be VERY CAREFUL WHEN FITTING KNOBS WITH GRUB SCREWS TO POTS WITH SPLIT SHAFTS!!
I wound up rebuilding the pot head using some mild steel and superglue to fill the split back up, then put the other half back on and filed the thing back into a good shape to take the knob. It's a good permanent repair and thankfully the knob is straight. By eye, no less!
Results? She plays excellently - always did - but now even the passive tone is better (new pots). With the OBP turned on I've got great capacity to dial up excellent tones: the bass can now produce that excellent Nathan East type tone from "get lucky", which it really couldn't before. The old preamp was all bass, very little treble.
I'll be playing her more in future: she's giggable again!