| Hi BassMan86, I had the same thing happen Monday. I noticed it came and went. After completely going through everything on the preamp with an o-scope, no joy, I started tracing each and every wire for either a cold solder joint or a broken wire.
I finally found the ground seemed variable on the bridge. Once I removed the bridge, after removing the strings, I found the cause.
There is a single ground wire (~22ga, black) going from the left side of the preamp (labeled gnd on preamp) through a hole in the front of the cavity to under the bridge, not to be confused with the shielded pick up wire on bottom of the preamp labeled piez & gnd.
The end of the single ground wire was shoved into the screw hole for the wood screw under the D string, and the wire was not making solid ground contact with only a few strands of the wire present. I stripped about 1/4 inch, of the wire placed it back in the hole (don't like this method at all) and reset the bridge, reinstalled the D string bridge screw. I verified the fixed by plugging into the amp, (sans strings) and increased the volume to max. No white noise... now the bass is almost perfectly silent. Then I reinstalled all of the three other remaining bridge screws (1 under each string)
While doing all of this I noticed three things I would recommend "fixing" and which I will be doing this weekend.
First, there is only foil shielding on the back of the plastic housing cover. I plan on putting foil shielding on the entire preamp housing area cavity.
Second the preamp is just floating in the cavity, actually mounting it would prevent it from shorting out 18v on numerous metal tabs on the plug, pot & bread wrapper.
Third, replacing the metal "bread wrapper" cable tie with a proper plastic gleeble (small cable tie), which I already did.
BTW, I was actually impressed with the overall design and quality of the buffered preamp and the circuit board. A few things worth mentioning is that there are solder pads on the preamp for external gain, tone, bass & treble pots, which could be installed relatively easily. (All while voiding the warranty I would suspect.)
And on the back side of the circuit board there are miniature raised mounted gain, bass & treble trim pots. | 
