Instead of the traditional gain-settable integrator, use a fixed gain double (leaky) integrator, and 2 variable feedback. 1 for cutoff frequency and 1 for Q. Would this work? I'm thinking: what about driving the input diode of the LM13700 directly using the filtered PWM? As long as we limit the current this should work just fine. EDIT: nope, it's not a diode.