If you do start looking at graphs of headphones you have to interpret them differently than speakers. Perceptual research into headphones shows that an ideal ruler flat frequency tends to be perceived as unnatural, bright and in some instances harsh. Since the drivers are touching the ear and are so close a gradual roll-off starting at 1khz-2khz is ideal.
If you look at a lot of high end cans like the 650's they have a lot of things in common - little to no bass roll-off, a slight rise between 100hz-400hz and a gradual roll-off starting at 1khz-2khz. This is sort of what is perceived as sounding "natural" in the headphone world.
