What you used has a big flaw - the preamp input is left floating ungrounded and that's the source of the pop.
As far as the mid-shift switch, I have no idea as I'd never used an Aguilar preamp (I'm guessing you have an OBP-3). However, the mid switch impacts the preamp internally and has no connection to the ground.
Personally, I'd go with a push-pull on the mid knob just for the sake of having a smaller footprint (fewer separate controls).
Squier VM Bass VI club member #19
Last edited by Stealth : 11-02-2012 at 10:49 AM.