> Any headphone/earphone can be EQed to "precise and neutral" if the distortion is low enough.

EQ can help a lot, but there are limits. The response curves you see are usually very smoothed and they’re only from one sample. The peaks and valleys and resonances can be narrow and can differ from unit to unit or even more from one manufacturing run to the next.

The response is also a function of the interface to your ear and leakage to the outside world. Some headphones are more sensitive than others.

I have a few high end headphones and I’ve tried a lot of the published EQs for all of them. In theory they should sound like they have the same frequency response but they still sound very different. Closer together when EQed, but you can tell they’re not the same.

I am an EQ proponent, but doing EQ on headphones doesn’t level the playing field so much that it only comes down to distortion.

The best case for EQ is in a sound controlled room for speakers when you can sit in the sweet spot and reflections and room effects are not a problem. Set up multiple sets of decent (distortion free) speakers in that environment with a moderate EQ and my listening skills aren’t good enough to identify differences most of the time.

True, and that's why you have to EQ by ear for best results. But even that won't be perfect, because the frequency response changes depending on exactly how the headphones are seated on your head, or exactly how the IEMs are inserted. And if there's foam in the construction, it will gradually compress over time, also changing the frequency response. It's still likely to be better than what you can get from loudspeakers in a typical room.