I just took delivery of a used fretless Carvin BB75 that has two J style pickups and a preamp with a three band eq and sweepable mids. My problem is that the bass has a noticeable hum. It hums all the time. On a gig last night it seemed to be worse when in active mode as opposed to passive but today at home, the hum was just as bad whether active or passive. The hum does not change when I go thru different pickup positions; It's the same whether neck, bridge or both. Frequencies boosted or cut by the eq, boost or cut those frequencies of the hum. I checked both batteries and they are both over 9 volts. I also took a probe and poked around in the control cavity. It was dead quiet. I could not make any crackles or pops happen. Is this common with Carvin basses? Is it fixable or do I need to replace the preamp and/or pickups? I ended up getting this bass cheap do to a supposed surface crack near one of the tuners. Initially it was just to try the bass but I'm really starting to like it so I'd like to get the hum fixed. Thanks.