the old ones were 4 conductor humbuckers. The new ones only have a hot and ground coming out of them, but they are humbuckers as well. THe noise is the same as previously, only much louder. I hope the new preamp will solve the issue.
Something is not quite right here.
First you have to decide if the hum is single coil hum or normal electric hum. The usual hum is the hum that goes away when you touch the strings or grounded metal. If touching metal does not change hum levels then shielding isn't the issue.
Single coil hum, on the other hand depends on the orientation of your bass. If you can take your bass (not playing it) and just move it around at all angles and find an orientation where the hum "nulls out" or disappears, then you've got single coil hum. You said in the original post that this happens when you are perpendicular to your amp. Note that amps are usually a strong source of single coil hum (especially if they have large power transformers).
It sure sounds like you've got single coil hum. If that is true than that implies that your "humbucking" pickups really aren't bucking hum at all for some reason!
If you want to make sure and eliminate all other variables wire the bass passive and see what it sounds like. just find the wire that goes to "input" of the preamp and lift it off of there and lift the "output" wire off the bass jack and temporarily connect the input wire to the jack. Then the pickups are wired directly with no preamp (only works if pickups are not active which it sounds as if yours are not)
If you still get hum in passive mode and again if it depends on bass orientation then you've got a "humbucker" problem. They either are not wired right or are defective. If the hum is gone passive, then the problem is clearly with the preamp.
I'm making a couple of bold assumptions here that I'm not sure are true. Which is that the pickups are passive and the volume controls, blend, ect. are all passive circuits ahead of the preamp. If the preamp uses an active volume control (like my Fender feh!) Then this operation is not so simple.
It shouldn't be too big a deal to narrow this down a bit.
