Check to make sure the pickups, all three, are well glued. Imagine one of those pickups with a free surface. It may act like a loose trim on the side of a car going 80 down the highway, flapping in the breeze.
Also, you can pick up or may have a DI box with a ground lift, that may help?
Then there is the phase thing. I like you know little about electronics, but do know your pickups can be out of phase with sound amplifiers, and by flipping the phase switch it can be in or out of phase.
Where is Rick T.?
One reason K and K now recommends the mini pure westerns is they have less output. The standards you mention put out a bunch of excess signal that basically goes unused.
One last thought about pickups flapping in the breeze. I had one Martin Dred with little space to mount the Standard K and K pickup. It had a little overhang of the bridgeplate when in position for gluing. After consulting with the owner, I added a 1/4 inch additional bridgeplate across the front of the existing bridgeplate. That way, no piezo discs out there doing their own thing. The were all glued in securely, and never had a problem with it.
This is the first time I've heard of a problem with a k and k pickup. I can make a recommendation, undo K and K's wiring scheme and test all three pickups indivivually. IF only one is bad, you could isolate it that way. A pain to do but having the guitar problem free may require it.
Good luck finding the problem. I find myself humming occasionally.
