I analyzed the API 550A curves and rebuilt them in Max for Live for you!
I’ve been working on a Max for Live EQ inspired by the behaviour of the API 550A. The goal wasn’t to make a clone, but to recreate some of the things that make those EQs feel different from standard digital parametrics:
stepped frequency/gain workflow overlapping musical bands oversampling
I spent some time comparing curves and behaviour against references and tried to translate that workflow into an M4L device that feels good directly inside Ableton. Just look for anychannelaudio!