Hi gavin thanks for looking into this!
I think you are right the filters do indeed change the sound even when in an open position, this is what I already noticed with my simple expression pedal setup.
I already experimented with fades by feeding a boolean into a smooth object. So neutral is boolean off, filter on means boolean on, and the smooth will give me the fade time.
The tiar/mux does indeed look interesting, thanks for the tip! As oposed to normal mux with and integer selector it does have float input for that. I'm not sure how that would work, I need to try this out.
I think even with fading there will be noticeble change in sound. the best thing would be to have filters that go up all to 20k as discussed here