I got here, eventually.
Part of the reason why this design works is the really lo-fi FET section. There'll be lots of second harmonic, and to have that nonlinearity in the impedance to ground in a multipath filter will cause all sorts of artifacts. When I designed it, this crudeness could be put down to inexperience. I should have put half of Vds on the gate, but I didn't know about that then.
Endangered Audio in North Carolina are sending me one of the Gristleizers they've made. They tweaked it a bit, putting on the known improvements of an external BIAS control, and a LED following the LFO. There was some ticking trouble, which I've always believed was down to layout, but they've done their own fix for that.
I shall have a good old poke around with that and see what the distortions are.
Roy Gwinn