I had interest to high order shelving audio equalizers, and I implemented one of them.
It can be found on https://github.com/thedrgreenthumb/orfanidis_eq.
Also, the second problem was to implement it like a some audio plugin.
I found that the simplest way to do it is integrate it into Calf project.
Let me share results (see attached).
The patch can be applied to commit with number a3b4f7dc22...(Crusher: fix UI crash) without merging.