Double Bass Wilson K4

[DoubleMIDI]

I doubt that grounding the strings to the shield would help with a piezo.

I rather think that one pickup element is not connected to shield or not connected well enough (perhaps not to the case of the piezo element).
Try a connection from the jack case (shield) to the pickup element case.
Best without touching anything (use clamps).

 

[Debasss]

Thanks for the reply, I spent quite some time on it and I finally found the solution.

The problem probably comes from the tailpiece of my bass which is not wood made.
I first tried with the small wire suggested but it wasn't very convenient to attach and the results weren't stable.
After a couple more observations I figured it out by simply attaching the mounting jack to the top of the tailpiece, in contact with the piece that touches the bottom of the strings. With this solution it becomes extra quiet!