I feel like your understanding of how we perceive sound is wrong, but I have neither the knowledge nor time necessary to argue with you. All I can tell you is that a speaker and IEM with matched frequency response sound completely different. I'm sure you'd agree that I'd be able to accurately tell you for any item I have never heard if it is a speaker, headphone or IEM, no matter how you EQ it as well. I see sound in shapes and colours and it is just completely different things.

I don't feel like you engaged any of my arguments. Do you think an open headphone and an IEM are going to sound the same if you managed to match FR?

I am not using any mics. I am using measurements and my ears. But the differences are so big that I can't imagine they can be up to 1db more or less here or there. Just as an obvious example the bass of an HD 600 sounds completely different than that of a Sundara when both are equed to flat. The HD 600 sounds like it is struggling and a large part of the resolution in the mids/treble is lost. Or I'm sure you'd agree as well, that an open and closed headphone sound different, even when they have the same FR. Or even more extreme, an IEM and an open headphone are going to sound completely different, even if you manage to match FR.

I never know if people are just lying about these things, I have god-tier ears, or I am doing something incredibly wrong when eqing headphones to match each other. To me the differences are night and day. If I EQ my HD 600 and AKG k612 to the same FR, I can clearly tell the superier soundstage and imaging of the AKG and the resoultion and timbre of the Sennheiser apart. Even when I EQed my DT990 and 880 to match each other I could clearly tell them apart in a blind test. With Amps it gets very difficult to tell them apart while switching between them, but I am imagining differences in longer sessions. I might be hallucinating on that one, or it's true that everything sounds similir when switching quickly between things.