passive mid-cut

Searched thru various threads, esp. the bass-cut thread, to find out if a reasonably simple and passive mid-freq cut schematic was hiding in these pages anywhere. Mid-cut for a bass. I've seen a mod you can buy (Rothstein) using a push-pull pot, a couple of resistors, a cap and a small inductor coil(?)

I know one of you guys will understand better how this works and how to wire it.
Cheers!