You can't compare a perfectly straight line and say the headphone sounds dead neutral because of it. You have to compensate for the ear's frequency response-- the upper midrange and lower treble tend to get amplified from a headphone, so what measures dead flat for the compensated graphs is actually on the bright side of neutral. Both headroom and Tyll mention this, but take a fr graph, and draw a flat horizontal line out to roughly 500 or 1000hz, then make it taper down gradually -10db to 20khz, and that's a more realistic representation of neutral. Of course everybody's ears hear things differently because they're shaped differently, but it's a better way to find a neutral headphone compared to just drawing a flat line. Also this is just my personal method of flatness, so don't take it as universal.
Also to note that perceived frequency response-- especially in the bass range, also differs drastically depending on the amount of ringing and non-linear distortion in said frequency range. Purrin brought up an example about the TH900 vs a Denon headphone. The TH900 measures to have a huge bass bump compared to the Denons, but it has way less non-linear distortion in its bass, and it decays faster, so it is perceived to have less bass.
Also to note, is that you might not want a perfectly flat headphone/speakers for mastering, you might even want something on the brighter side of things so you can more easily get rid of flaws in the recording. NS10 is a good example of this.
Here's my take on the TOTL headphones:
Lower bass rolloff is much more apparent in actual listening, this is a limitation of a dynamic driver open headphone. Its lower treble is on the brighter side of things, which is easily noticeable in listening. Its midrange is so gooood.
I've not heard this, but may people describe it to be extremely flat and has the best treble reproduction of all headphones.
This headphone is described to have the best bass reproduction and the most accurate midrange timbre of nearly all the headphones on the market, but many people also don't like it because it's lacking in extended treble after 10khz that can provide 'air' and 'sparkle' to the music.
Edited by TMRaven - 8/6/12 at 11:17am