I'm not convinced, the Nothing Ear spatial audio effect (which is pure postprocessing as far as I understand, since it's independent of codecs and containers) sounds a lot more realistic, and I doubt Nothing has the best algorithm in the field

I'd be interested in comparing it with the Nothing Ear effect. If it's linear (and time invariant), it should only require playing a test sample through it, recording what happens, and using an automated script to extract the responses.

As for the effect, I was trying to get a particular effect I'd liked using earlier, rather than the most realistic spatial audio algorithm. The earwax effect mentioned elsewhere in the thread also sounds quite close to what I wanted. Posting this also worked as discovery, getting me some good suggestions for other effects to reverse engineer.