The circuit below uses a stereo, three-input mux as a DP3T switch where the "left" section chooses between two DC sources for the filter's Q input, and the "right" section selects filter or bypass. The idea is to have three switch positions: High-Q, Low-Q, and OFF. However it won't compile, throwing an "illegal feedback" error instead:
If the output and Q functions are brought out to separate muxes as shown below, it compiles fine:
In neither circuit does the filter's Q input ever come from its BP output; these signals are handled by separate mux sections. The two circuits are functionally identical -- the only difference is the button choice presented to the user. Thus, I don't understand the illegal feedback error that the top circuit generates.