This is Yves Usson's take on the Steiner-Parker filter topography. It can be a very harsh filter if you treat it wrong, and the resonance on my one can be a bit touchy -- there is a very fine line between mild, and OHGODMYEARSAREBLEEDING!
I wound up replacing the resistor in the resonance part of the circuit with a trimpot, to try and get a bit more range out of the pot on the front panel. Seemed to help. The panel for this was 15HP, from memory, chiefly due to the rotary switch I used. I didn't bother matching any of the diodes I used, and I seem to have gotten away with it.