Keep in mind that rapid A/B switching tends to erase differences. That's a familiar problem for people who go to big meets where lots of headphones are available for demo and they try out multiple headphones/IEMs in the span of a few minutes; everything starts to sound the same because our brains don't have time to get acclimated to any one sound. It's a common phenomenon that headphones that sound good during a short demo at a show don't sound as good in the long run, because the sound qualities that make it stand out against the 5 other headphones the listener just heard make it too sharp or too unusual in a normal listening environment.

I've gotten headphones very close to one another via EQ as well (though never quite exact), and resolution is something that to me is mostly linked to FR. I actually don't like the term "resolution", and I prefer the term "tonal contrast", which I think is a more descriptive term for what I hear. Contrast is what allows me to differentiate between different sounds (similar to how visual contrast is a key part of how our eyes perform object recognition and differentiation), and to me, "resolution" is how easily I can distinguish different instruments and sounds. It's a very fine-grained balance between different frequency ranges (plus lack of cumulative distortion from the lower registers that might interfere with the presentation of the high frequencies) that produces the correct contrast for good "resolution".

Soundstage is the main thing that cannot be easily replicated via EQ, and that's because the headphone's interaction with your HRTF matters a lot for that. Even Dr. Sean Olive, possibly the foremost expert on headphone FR measurements, has said in the recent interview with Resolve and Crinacle that FR measurements aren't everything, and don't measure the spatial qualities of a headphone. I could not EQ my Sundara to have the same soundstage size as my Shangri-La Jr, even though I could approach its resolution and overall sound. But the placement of sounds is something I could not reproduce via EQ, the SGL just sounded more spacious. The physical sizes of the drivers are different between the two, so the wavefront that hits my ears is different, so the interaction of my ears to that wavefront is different as well. The X2 and the Sundara have similar sizes and shapes I recall (I only briefly owned the X2 years ago), so soundstaging differences should be less pronounced between them.

With EQ, I've noticed regions (different for each headphone) where the magnitude of EQ applied doesn't match the magnitude of the perceived change in the sound. I've noticed places where 0.5 dB makes a noticeable difference in the sound. I've also seen cases where I boost a range by 10 dB and it does nothing to erase a dip in that range (that's usually with closed-back headphones with undamped earcups). I've found that even if I can EQ headphone A to sound like headphone B, that's no guarantee that I can do the reverse, make B sound like A.

There's also another aspect of sound that can be produced with EQ, but not via standard EQs (graphic or parametric). I've started using dynamic EQ, which boosts/cuts a frequency band only when dynamic swings occur in that band, and that allows me to add the "punch and slam" of macro-dynamics into a headphone. So in a way, dynamics are FR too, but not in the FR that you can easily see in a graph, it's sort of "instantaneous FR" if you will. I've heard of "attack measurements" at SBAF and also of impulse response overshoot as metrics for dynamic performance (more overshoot in the impulse response level means more slam), but either way it's not something that you can easily see in the standard FR graph yet has quite noticeable effects on the sound.

My overall view is that I don't agree with people who say "FR is everything" and mean that you can just look at your usual FR graph and immediately know how a headphone sounds. Even experts like Dr. Olive who specialize in those FR measurements don't hold that view. I take the view of "momentary SPL at the eardrum is nearly everything". I leave open the possibility that part of what we perceive is not eardrum-related, like maybe there's an effect perceived by the skin of the inner ear canal. There's also the fact that our brain doesn't work on SPL, but on loudness, and those two do not correlate exactly.

I also like seeing CSD plots, as I've read that at higher frequencies (>2KHz and with some effect down to 500 Hz), our brain doesn't maintain phase lock with the incoming sound wave, but instead the perception process is triggered by the waveform envelope. I've heard it explained that outside of the phase-locking region, the brain "batches" sound in time, and perceives the total amount of sound occurring in each batch as its loudness. Longer sound = louder. My understanding of this is that if a peak in the FR (above 500 Hz or so) has a long trail in the CSD plot, that peak will sound louder than the plain FR would imply. You've already seen the demonstration of how EQing down a peak also cuts the CSD trail, so the headphone "double-dips" from the EQ, not only is the peak gone, but the amplifying effect of the CSD trail is also gone. I suspect that may be why I notice unusual effects when EQing, I'm changing the FR at the same frequencies where there is a significant CSD trail, so the effect is either muted or amplified. When I've done my measurements, it's usually the case that regions with odd EQ interactions also have longer trails in the CSD plot. Psychoacoustics is a really interesting field that I wish I studied more in college (I studied electrical engineering with an emphasis on computer microarchitecture, so outside of a few audio engineering classes, I never went too deep into that subject).

The post I was waiting for. Thanks for sharing your experience man. Mine is similar, because it's about a year and a half that I own a pair of X2HR and some other headphones below 200 bucks, but it's only a week since I've finally found an EQ profile capable of making them sound warm and non fatiguing, without killing treble and details. I can share with you my EQ profile, it would be interesting to hear your take on it since I'm very proud of it hahaah.

Which software did you use to render the output of the microphone into a frequency response?

No you didn't

Can you do something like this - turning Hyundais into Teslas?

i will be more impressed if you can make a HD600 to a HD800 or even to a HE1000 v2

Sundara now irrelevant.

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.

If you can't hear grain on the Phillips you need to work on ur hearing