A few questions:
1. Would the sound of such a headphone be desirable?
2. Do frequency response graphs exist for headphones such as the HE90s and Stax 007s?
3. Can speakers come closer to this?
1. Probably not, since it would not actually sound flat.
You have to start by asking how do you intend to measure flattness, i.e. where do you put your mike. The frequencies coming into the ear, increase in volume between about 2 and 4 kHz due to things like resonance in the ear canal. So if your phone measured flat using a small probe mic at the base of the ear canal your are ok, you do have a flat response. Measured anywhere else or by any other means, no.
2. Probably, but note the reservations in 1. above.
3. I am not sure that speakers can in principle be given a flatter response than phones, considering that they are subject to room resonance effects, that in smaller rooms can be quite whopping. Generally these are in the lower frequencies and give some pretty nasty bass effects if the speaker previously tested flat in a large room.
I use an equalizer and mic set-up with my main speakers and the flatter I can get the speakers to perform the clearer the sound gets. However you may need to reduce the high frequencies (in fact my old dBx equalizer has a dedicated roll-off button for this purpose) because flat sound is too bright. This is probably the result of 2 factors related to high frequency attenuation in air. First the microphones used in the recordings are very close to the source, closer than a listener would normally be, thus making the sound too bright. Secondly your speakers or phones are very close to your ears similarly increasing brightness.
Then a lot of rock/pop seems to have a high frequencey emphasis, probably added in recording making them bright.