I got it to work! I had to do two things: 1. Replace C30 (the cap on the first integrator) with a very small value, 0.5 nf or smaller. 2. Bypass the first section of CR12 (the frequency control) so it's always 0 ohms.
I tried lowering the reistance of R104 (on the input of the first integrator), but if you lower the value too much, the circuit misbehaves and you get DC on the output. You can't remove or bypass the cap either.
Basically, you're fixing the LPF frequency above 20k. I'm not sure what would happen with an even smaller capacitor, it might cause misbehavior. A more elegant solution would be nice.
The result is a good sounding, sweepable, 2cd-order shelf. It has the same frequency range, and it can easily be made switchable. I still have to do some pink noise tests and try it with a Q pot. When I do I'll post the results.
