My Tone Wicker Muff is powder coated. The position of the spring connecting the circuit with the housing was also full of powder coate. So i pulled away that paint to connect minus to mass correctly.
Now the hiss is gone, when the Tone Wicker switch is engaged.

With egaged Tone Wicker there is some similarity in tone with the old Jordan Creator, a Triangle clone from ’71, when you lower down the volume a bit.