How hard would it be to hack a state variable filter wich is pretty diy open to have a summed post crossfader output instead or additionaly to lp,bp,hp outs? Like a blend between these filter modes.
Easiest way would be to feed the outputs to a vc mixer. Try it first with a non-vc mixer to see if it is actually interesting.
Sure a passive 3-1 attenuator would also work but would add a blend/crossfading knob
I think the Nonlinear Circuits Noiro-ze has something like the cross-fading you’re describing, though how it accomplishes it is a bit mysterious to me just glancing at the schematic.
It’s kind of the opposite topology of what you’re taking about though, since is has individual LP, BP, and HP inputs and 1 output
Cool concept. From the single demo it doesnt really sound very nice to me i suppose.
Was thinking of hacking a cinnamon. But maybe a wet/dry or simply 3-1 attenuator does the thing i think of. But the idea was more to travel through filter behaviour with 1 knob
There’s a somewhat easier to listen to demo of how the filter sounds here:
I haven’t tried it myself, but it’s on the list for future diy builds for me for sure.
The only way I can think of to accomplish the one knob thing off the top of my head otherwise is a Frames, which is a bit of an expensive cross fader.
Maybe a clever Cold Mac patch could work too
Worth noting the Oberheim SEM filter, which has a blend between LP and HP where Notch mode is in between.
Also the Fritz Threeler, though it’s switched and not crossfaded, but has a wealth of combinations of single stages producing multiple outputs and modes.
Phase relationships between the outputs will be relevant in what you end up getting, worth experimentation with an external module before hacking on a PCB 
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