I have a Washburn T14 bass and I've noticed that with the J pickup there seems to be some noticeable noise when the volume is turned up, just like a static hum (even when I'm not playing anything). This doesn't happen with the P pickup (there's no hum when the J's volume is all the way down), so I'm wondering if that's a normal trait with the J pickups or if it might be broken. I also noticed if I have the P at full (or any volume really), as I turn up the J's volume it seems the overall volume actually gets a little lower. Again, I'm not sure if this is a normal property (perhaps one is canceling out the other) of it the thing's just busted and messing up the signal.

I have a Washburn T14 bass and I've noticed that with the J pickup there seems to be some noticeable noise when the volume is turned up, just like a static hum (even when I'm not playing anything). This doesn't happen with the P pickup (there's no hum when the J's volume is all the way down), so I'm wondering if that's a normal trait with the J pickups or if it might be broken. I also noticed if I have the P at full (or any volume really), as I turn up the J's volume it seems the overall volume actually gets a little lower. Again, I'm not sure if this is a normal property (perhaps one is canceling out the other) of it the thing's just busted and messing up the signal.

Unfortunately, the hum you're hearing form the J pickup is normal for this kind of single coil pickup. The pup is sensitive not only to the iron in your bass strings, but to any stray magnetic fields in the immediate vicinity, such as the power supply transformer of your amplifier or other gear, computers, etc. The reason you get little or no hum from the P pup is because it's two halves literally become a hum-BUCKER (or more accurately a hum canceller.) This is accomplished by winding one of its halves in the reverse direction from the other, along with reversing the polarity of the magnets in one of the halves. The bass remains in absolute phase, while the noise picked up is phase cancelled. All "humbuckers" have this characteristic. Unfortunately, it is not possible to do this phase cancellation trick with single coil pups, so the noise will sometimes be there. There are humbucking J pickups available, but the trade-off may be that you do not prefer the sound of them to the single coil.

Regarding the slight volume drop, this is most probably due to the phenomenon of "loading," where the output of each pup is slightly dragged down by the influence of the other when both are turned up. But if you feel the volume drop is WAY more that what seems reasonable, it's possible that one of the pickups got wired out of phase with the other, though in all probability this is very unlikely. (This problem has nothing to do with the normal phase cancellation the P pickup does for cancelling noise.)