If the EQ profile to match them didn't introduce any audible distortion then they should sound almost identical. I would even argue you could make a headphone sound almost exactly like a speaker. I heard SMYTH Realiser A16 does just that very well.

Having said that, your perception of the sound might change when using an open-back headphone since you can hear ambient sounds in the room. However if you were in a completely silent room which is pretty much impossible to achieve, you should percieve them identically as well.

I used Room EQ Wizard.

Well, if you're not using any mics you're not really matching their FR. Your HRTF interacts with different headphones differently and there's unit variation. Measurement graphs online don't tell you how your ears hear a headphone.

This. The only two notable imperfections about this method are these two initial assumptions:
1- If two identical sounds enter the ear canals, they will sound identical to the ear drums.
2- When more than one particular frequency is played, how the headphones react to the input will not deviate from their FR.

Thank you very much for sharing your knowledge. I can't say I am qualified to fully understand everything you said (and it's even harder to do so because English isn't my first language) so I have a few questions if you don't mind.

What exactly is "cumulative distortion from the lower registers"? That and FR are the only 2 things that determine a headphones' resolution/tonal contrast then, correct?

I always thought that angled/far drivers or different sized drivers have different soundstages because pinna changes the FR of the sound that is entering ear canals differently. Therefore spaciousness and directionality of the sound is mostly (maybe even entirely) determined by pinna and since in-ear mics roughly measure the sound arriving my ear canal entrance (so it takes the interaction between the headphones and my HRTF including my pinna into account), can we really not replicate soundstage using EQ? If we can fully match the 2 sounds entering my ear canal, my eardrum should hear identical sounds as ear canal will react identically to both sounds, should it not?

If I understand correctly overshoot in impulse response level is slam. So if an impulse of larger than intended amplitude is created it will have more slam and maybe will sound nicer but is it the intended sound? I'm not asking if it's intended by the music producer, I'm asking if it's closer and more accurate to the digital input.

I've heard from multiple people that most headphones are "minimum phase". I don't know what exaclty "minimum phase" is but what I understood from it is that in CSD plots, if there's a peak in FR there will be a peak in decay time and vice versa and they match well enough so that it is under audibility threshold so they don't matter. Is this correct?

I now strongly agree on the idea that "momentary SPL at the eardrum is nearly everything" as FRs don't show how the headphones react when more than one particular frequency is played.

So if a headphone can't keep up when multiple instruments intended to sound like they're coming from different directions are played, I think the sounds might bleed into each other and hurt the fine details of the sounds and the sense of directionality. Could this be what people refer to when they use the terms "separation, resolution, imaging"? And I have a feeling this is related to the attack and decay speed of the headphones, is this true?

Are you using in-ear mics to measure as well?

No worries, glad it helped :)

Yes, unit variation is definitely something to consider. There's even a very noticable channel imbalance on my unit for example.

Is the high THD the reason for grittiness, if not what is it? And why is it not observable in FR?

I use Equalizer APO. I'm used to the default interface so I never really tried Peace honestly. I listened to some stuff with your EQ profile and I measured it. It's certainly much better than default tuning. The harshness in 5-6kHz is gone, it even sounds a bit too recessed in that area for me but since you're sensitive to that I guess it's fine. I think the upper-mids are too recessed so I would recommend adding a 3000 Hz, 4 dB, 1.5 Q filter. If you don't like that you could try 2-3dBs instead for your preference.

Well I never claimed to have good hearing. I just said they sound almost identical to me. Can you explain what exactly you mean by "grain"? Maybe the slight difference in separation I hear is what "grain" is.

Sure I'd be happy to take a listen. I can share mine as well but I'm not sure if it would sound right to your ears or anyone's really.

I don't know how to share a txt file so I'll just paste it I guess. Here it is:

Preamp: -8 dB

Channel: R

GraphicEQ: 20 2; 50 -2; 80 -2.2; 170 0; 220 0.8; 254 0; 267 0; 300 1.7; 360 1.9; 410 0.2; 765 2; 808 0; 833 -1; 901 0; 1159 0; 1300 -1.5; 1655 -3; 1764 -5; 2035 0; 2096 0; 2137 1; 2224 0; 2283 0; 2450 1; 2800 2.5; 3202 3; 3469 1.3; 3567 2; 3683 0; 3948 1.5; 4000 -0.8; 4100 -1.5; 4187 0; 4510 0; 4649 2.5; 4710 0; 4850 -5; 4955 -4; 5000 -5; 5158 -4.5; 5268 -7; 5352 -8; 5434 -9; 5538 -8; 5671 -8; 5906 0; 6019 3; 6189 0; 6490 -3; 7439 0; 8003 2; 8739 2; 9140 6; 9312 4; 9465 6.5; 9789 4; 10000 3; 10129 0; 10185 -1.5; 10244 0; 10423 2.5; 10581 0; 10630 0; 10696 2.4; 10763 0; 10828 -1.5; 10868 0; 10951 2; 11026 0; 11124 -10; 11266 -1.5; 11508 -6; 11585 -3; 11930 0; 12437 0.5; 13012 6; 13516 4.5; 13787 5; 14206 5; 14923 5; 15107 7; 16313 7; 17505 0

Channel: L

GraphicEQ: 20 1; 40 0; 50 -2; 81 -2; 170 0; 180 0; 250 0.5; 262 0; 275 -0.6; 292 0; 400 0.9; 433 0; 465 -0.9; 530 0; 760 1; 800 0.6; 842 0.6; 900 2; 951 0; 1019 0; 1197 1.4; 1413 0; 1650 -2.5; 1985 0; 2300 2; 2541 2; 3200 3; 4110 3; 4356 0; 4378 0; 4421 1.8; 4614 2.5; 4666 0.5; 4718 0.5; 4807 2; 4900 0; 5000 -2.5; 5096 -3; 5350 -7; 5426 -5; 5505 -4; 5590 0; 5672 -2; 5731 0; 6002 3.7; 7442 0; 7465 -0.5; 7735 0; 8288 4; 8631 2; 9005 1.5; 9424 5; 9799 0; 9890 2; 9966 0; 10060 1.3; 10097 0; 10125 -1.6; 10176 -1; 10219 -2; 10248 0; 10318 4; 10406 0; 10443 -2.5; 10506 0; 10659 -6.5; 10683 -5.5; 10732 -12; 10767 -10; 10825 -20; 10951 -5; 11092 -7; 11330 -2; 11527 0; 11645 3; 11708 0; 11776 -9; 11859 0; 11894 2; 11980 3; 12176 8; 12430 3; 12788 7; 14914 2; 15158 4.5; 15376 4.5; 15673 7; 15959 2; 16122 6; 16246 6; 16372 0; 16496 0; 16591 -4; 16739 -2; 17122 -3; 17223 -1; 17407 0; 17826 2; 17949 4; 20000 0