tell me more about the changs you made and the other pickup.
I just had the split coil tone pot replaced with a conventional tone pot. So, instead of being centred with each direction bringing in more of each pickup coil, it just rolls off like a P bass tone control. The bright toggle switch (or whatever it's supposed to do) was taken out of the circuit entirely. I also had my tech shield the wiring cavity a lot better than it was originally - but it still has more 50 cycle hum than any other bass I own.
Regarding the pickup, I emailed Kent Armstrong and asked him if he could make me a proper humbucker that could be plugged in and out like the other 2 interchangeable pickups (two coil DB and BB) that come standard with the ADA4 - after some delay and tooing and froing (Kent is not the best person I've encountered when it comes to email communication) I sent payment to his business in the UK, and he made the pickup and posted it to me. Off the top of the head I think he charged me $120 for the pickup - it's got the same 3 prongs and thumbscrew thread as the standard pickups, so it slots straight in and out into the pickup rout, just like the other modular pickups.
I love the neck and ergonomics of the ADA4 - but I wish it just had a meat and potatoes P bass pickup in it instead of the unusual configuration it comes with.