iir should replace in most cases. It doesn’t have audio rate modulation of the cutoff, being the main difference. Modulating biquad’s quickly can result in instability of the filters per https://music-dsp.music.columbia.narkive.com/r5TJ3GcJ/help-for-biquad-design-needed#post2, so the loss of audio rate modulation may not be a bad thing.
I’m curious what you are using iir for. Generally the instability of iir filters at low frequencies due to truncation errors makes them a challenge, though admittedly this should be better with the floating point of the H9000, than the fixed point of the H8000, depending on how iir has been implemented, and how closely it matches the H8000.
