DF doesn't imply a midbass boost at all. DF usually tend to keep the midbass to low-bass flat while slowly diving down to the sub-bass. FF is suppose to emulate speakers in a dead room so basically there would be slight 2db increase around 100hz to try to add weight to the kick and from around 50hz and under have slight boost to allow the headphone to give that sensational rumble from the lower octave(s). biggest difference between DF and FF is response past 10khz. lot DF headphones will have slight couple db dip at 10khz and rise around with tad peak from 14khz till around 18khz and take dive around 20khz so the headphone will provide that ''airy'' sound.

FF tends to peak 10khz a db or so and doesn't exhibit that much of a peak from 14khz and above so it will not be too airy and so forth. usually free-field tries to stay flat or takes little dip or peak here and there after 10khz.

sennheiser used DF but there own method. like how stax still believes in FF but their own method. there is only a couple headphones that followed a specific standard for studio use when it came to FF or DF use. for the he90 i can't say.

DF doesn't imply a midbass boost at all. DF usually tend to keep the midbass to low-bass flat while slowly diving down to the sub-bass. FF is suppose to emulate speakers in a dead room so basically there would be slight 2db increase around 100hz to try to add weight to the kick and from around 50hz and under have slight boost to allow the headphone to give that sensational rumble from the lower octave(s). biggest difference between DF and FF is response past 10khz. lot DF headphones will have slight couple db dip at 10khz and rise around with tad peak from 14khz till around 18khz and take dive around 20khz so the headphone will provide that ''airy'' sound.

FF tends to peak 10khz a db or so and doesn't exhibit that much of a peak from 14khz and above so it will not be too airy and so forth. usually free-field tries to stay flat or takes little dip or peak here and there after 10khz.

sennheiser used DF but there own method. like how stax still believes in FF but their own method. there is only a couple headphones that followed a specific standard for studio use when it came to FF or DF use. for the he90 i can't say.

This, just had a glance to compare my Senn chart to Max's and it says DF right on it.  Forgot where I had seeen it.   

Simply different design goals.  If you listen to extremely well recorded music w/ proper gear that isn't anemic or inherently bright it's quite fine tbh. 

Don't Tyll and HeadRoom use different compensation?

I thought part of the deal when Tyll left Headroom was that he'd still provide their graphs - it would be odd if he offered different compensation for their graphs than his.

I also hope he's compensated for offering that service.

It certainly looks like the perceived hearing compensations between his InnerFidelity graphs and the Headroom graphs are very different though, it would make sense if you think about it. He wants the Headroom graphs to emphasize more distinct FR features of different headphones so potential customers, even those with minimal understanding of audio measurements, could discern something. Also a much greater degree of FR curve smoothing is imposed on the Headroom graphs, for the same purpose I'd think.

If this assumption is true, then the IF graphs are more accurate.