60 cycle hum on a P bass pickup

[PlayItAway]

Hi all,

I have a Stagg BC300-FL (PJ configuration) with upgraded hardware and electronics. The pickups are both Bartolini; the neck pup is an old discontinued model that seems to be quite like a vintage P pup, and the bridge pup is a newer epoxy-sealed J model.
There's also a Bartolini NTBT preamp with a push-pull A/P switch.

The neck pickup hums, but not the bridge pickup. Seems like the reverse would be more likely to happen.

I remember taking this bass on tour and getting super frustrated. The hum would usually change volume depending on how I moved/angled the bass. IIRC, it made no difference whether or not I touched the strings.

The weirdest thing was just recently, in my practice space. I have a few different outlets in the room. My all-tube stack was plugged into one outlet, and a small portable speaker was plugged into the other. I noticed that whenever the speaker was on, there was a 60 cycle hum that would come through the amp - but only if the blend knob had signal coming from the neck (i.e. P) pup. If only the bridge (J) pickup was selected, there was no hum.
Once I turned the speaker off, the hum was gone no matter what.
In this scenario, touching or not touching the strings made no difference.

I haven't tinkered much with this bass, and wouldn't know if it would be shielded under the paint

What do y'all think the issue is? Grounding? Shielding? Polarity? Gremlins?

Thank you very much

 

[Jazz Ad]

I'm not aware of a Bartolini P model ever getting discontinued. They're all available from the very first 8S.
The 2 parts of the split may be wired out of phase. You wouldn't hear it but it would result in hum.

 

[PlayItAway]

I'm not aware of a Bartolini P model ever getting discontinued. They're all available from the very first 8S.
The 2 parts of the split may be wired out of phase. You wouldn't hear it but it would result in hum.

The pickup was given to me by a friend. I don't have the case it came in anymore, but it was a clear plastic case, not like the cardboard boxes that my new Barts have come in. I seem to remember it saying Bartolini.
Anyway, it has 4 exposed pole pieces per half, just like a vintage P pickup.

Could the problem be fixed by simply turning one of the halves of the pickup around in the cavity?

 

[Jazz Ad]

All older Barts used blades, not pole pieces and they never were exposed. They also say Bartolini across the pickup. So unlikely to be a Bart. A pic would help.
If phase is the issue, rotating one of the parts won't change a think. You need to reverse wiring on one of them.

 

[Groove Doctor]

Photos of PUs please....