It’s remarkable how stuff changes when it’s correctly grounded.
(I’d been trying for ages to figure out what was wrong with my Barton Sallen-Key VCF, a design very similar to the René Schmitz MS-20 Filter and therefore, I guess, to the LMNC Performance Filter. Finally I emailed Barton and he made some suggestions, one of which was: Check the grounding on the tip switch of the HP input jack. I did, and found it wasn’t. Grounded. I’d completely forgotten to make that connection. Touched a screwdriver between ground and tip switch and everything started working…)