As far as frequency graphs are concerned I prefer mine to reflect something akin to the equal loudness curve so that something closer to the following is
PERCEIVED. We should all know that lower frequencies are more difficult to hear, justifying the bass increase. But if your ears can pick up bass very well, then it isn't.
Of course with the caveat that the high frequency range on headphones are never flat. It will simply depend on individual hearing and experience.
But. regardless of what the graph looks like, I believe what we hear that makes us classify headphones as bassy, mid-centric, treble frieindly, or neutral, is the decibel difference between the frequency ranges with reference to the equal loudness curve. I don't know of that has been theorized before, but it is my own observation.
Take something like a Denon AH-D2000's frequency response graph and a Senn 650's FR graph. Notice on the low end that the D2K's stay even till about 20Hz whereas the Senns starts changing closer to 100Hz.
Similarly, observe the range between 1KHz and 3KHz. Whereas the D2Ks have about a 10db difference, the Senn's change is much less.
But all of that is sorta off topic. It's just how I use FR graphs once I find a sound I like and note any changes I would prefer to have.
As far as just the range of frequencies heard is concerned and ignoring all other factors like detail, soundstage, driver capabilities/hindrances, etc, finding an inexpensive pair of headphones that can produce the sound of the HM5s as well as the bass/subbass of the D2Ks is ideal TO MY EARS.
But in manveru's case, that's too much bass
IN CONCLUSION.
Hearsay is hearsay. One man's experience is different from another's. The only way they could possibly be the same is if they share more than quite a bit in common. In the audio world, that common thing they must share is how they hear sound. Experts are cool. But ultimately,
YOU DECIDE.
Also note. I could be wrong. I could be right. Part with it what you will. Theories are useless when faced with the real thing.
Sorry for the rant.
And yes. I own the D2Ks and the HM5s and have auditioned the Senns for several hours.