I think the major flaw with headroom measurements is that they use the same HRTF for all headphones, regardless of design criteria. Ideally, I think they should be also providing measurements involving moller's compensations for diffuse field, and other compensations for free-field (even if they use their own head, I don't mind ,but that's harder work).
Speakers change their sound based on the sound field, so I think that removing that variable is a false mechanic, even if you are using direction independant compensations.
I also think headroom has made some erroneous compensations for canalphones based on my listening experience and what various companies (i.e. etymotic) tell people. But that's just a guess and there's no real way to find out without raw data.
My perfect system would be to have raw data provided for the headphones, and have the ability to plot the HRTF, and Moller et al. findings. And then to subtract them, if you wished. But that's a hard ask.
Edited by MrGreen - 3/15/11 at 7:40pm