Hmmm. Still, the fact one bass does it and one doesn't suggests that the bass is the issue, not the pedal. Might be worth getting a technician to look at it and check the shielding and test the pickups - might be highly microphonic and causing feedback.
Actually, does it do it with the volume on the bass turned right down to zero? Would definitely suggest a problem with the bass.
Attitude II SFG; RBX-JM2; RBX4-A2; Thumb 5 BO; Corvette Std fretless; LMII; MB 121H; Nova Dynamics; EBS Billy Sheehan; BEQ-50; LS2; BSW; MXR Bass Fuzz Deluxe; Pitch Black; PT Jnr.