Limegreen line represents the "average frequency amplitude". I don't find any use of taking into account below 20Hz and above 20kHz as it may just skew the results, especially for open headphones which usually lack bass extension versus closed.
On V6 it has probably among the most flat measuring headphones I've seen with the most neutral "tonality" I've seen, probably why it's found in so many recording studios. Due to the slight dip after 50Hz as well as around 6kHz area (but should be mentioned Tyll's measurement almost always shows a dip in this area so it may just be something limited to this specific equipment/test procedure of his and might even be quite straight in reality which is my guess due to almost all headphones showing this dip) the line should be drawn slightly under the rather straight FR plot in the 50 - 5kHz or so range. Well if you want to be extremely picky you could perhaps make the line possibly ever so slightly upwards tilting due to the bass extension roll-off and 9~10kHz spike but that would be such a small tilted angle I just preferred drawing it straight in this case. Anyway it looks like there's like ever so slight emphasis around midrange and even smaller so in the 50-150Hz or so range but from my experience a 3dB variety in the FR graph isn't really going to stand out and will still sound pretty flat to our ears, might be more closer to 5dB or so variances when it becomes noticable, the peak at 9~10kHz is probably slightly noticable. Anyway this is probably the most flat measuring headphone in its price class by far, with only a slight roll-off below 50Hz and above 10kHz (I'm not convinced enough about Tyll's 6kHz dip as it's so frequently showing up and I've seen some other plots where it wasn't the case either so)
HE272 then again has a slight warm tilt in tonality (line is tilting forwards) versus the V6 with a more significant midrange spike in-between the 1000 - 2500 Hz range which is most likely audible for a more experienced ear probably with certain female vocals for example. It also has a relatively similar spike around 9kHz as V6 (well most headphones have a slight emphasis in this area and this is more the norm than an exception and a 5dB spike here isn't anything unusual at all). But yea it's fairly flat in the bass range overall and has an emphasis in the 1000 - 2500Hz range and recessed/rolled-off slightly around 2500 - 8kHz range and above 10kHz.
Edited by RPGWiZaRD - 8/6/12 at 8:29am