Do the pole pieces of the Wal pickups have a pretty narrow field? I notice on all Wals when doing string bendings the volume drops when bending up. Never noticed this on other basses. You can check that on your Wal. I was wondering why that is?
You might have bad pickups in your Wal too. Have you checked the control cavity for loose connections? It’s amazing what vibrations can do to fine connections.
The reason I ask is I had volume reduction in one of my Wals and the correction was in fact a loose wire.
A bad battery could be another...always great to check the 9V battery voltage.

