What does it take to design a 4th order band-pass filter with -24dB/Octave?
After experimenting with winfilter, generating the following filters mimics the EFX isolators, amazingly :)


2 * Butterworth LP 2nd order @ 300Hz
2 * Butterworth BP 2nd order @ 300Hz/6000Hz
2 * Butterworth HP 2nd order @ 6000Hz


So each signal passes through two filters of the same type, for some reasons I don't know the cause of, a 4th order BP in winfilter simply doesn't work and produces garbage instead.

As a reference, here's what really helped me out in matching the filters properties, white noise recorded for each of the bands then guesses until it matches:



Surprisingly, I'd say the above filters match the EFX about 99%, the only differences are at the edges where the winfilter ones really trim down the frequencies much more.