Exactly. You hit the nail on the head. EQ is not magic. It cannot turn one headphone into a clone of another. It can't turn a mediocre or poor headphone into a high quality headphone. What it can do is correct *some* of the faults *some* of the time. It can't help with distortion, poor fit, wrong polarity, poor quality components or assembly, variations in FR response according to impedance and so on. Do people advocating this EQ and that EQ solution never even consider that their magic figures may become complete nonsense as soon as the listener connects their headphone to different amp or device with different output characteristics? If a headphone measures and sounds poor out of the box it doesn't deserve glowing reviews. And how brilliant is EQ that introduces distortion?
These EQ centred reviews have become ridiculous and imo the fact that audibly and measurably mediocre headphones are getting recommendations undermines and discredits all the assessments.
You make some good points about the unkowns with headphone measurements.
They are unlike speakers where in-room, listening windows measurmeents are easy peasy to take and validate.
That said using PEQ on speakers for quite awhile now I have come to the conclusion it is the single most powerful audio "component" in the chain once everything else is at least at a basic "okay" level of performance. There are still many variables but it is that powerful.
I am grateful to have him measure headphones so I can try to PEQ them with some basic starting place.
This is far better than the completely subjective reviews that have no measurements at all which sans any measurements are utter and total nonsense when it comes to hifi. (I like my reviews to have both objective data and subjective experiences)