in terms of expertise, i have none. (though i do read A LOT of books on music, speaker design, and audio) but anyone who's taken an elementary music class knows what timbre or tone colour is. it's what distinguishes one instrument from another. for example, if i play note on the guitar and play the same note on the piano, you can easily distinguish that they are different instruments despite the fact that the note is the same. although the fundamental tone on both instruments are the same, anyone who has taken physics will tell you that the overtones are different, and this is what imparts the difference in tone colouration.
this concept is also well known to anyone who builds speakers. an instrument's enclosure helps to add the overtones that cause colouration. but unlike an instrument, when you build a speaker want an enclosure that doesn't add harmonic overtones. you want the enclosure to be acoustically dead. this is the reason why speakers are made with MDF (which is wood made of compressed sawdust) or plywood. solid wood has grains that will add colour to the sound. as this applies to speakers, it is also pertinent to headphones.
it doesn't take much expertise to hear that the timbre isn't nuetral on grados. grados are well known to be coloured. grado labs even describes the grado sound as "warm harmonic color."
but that's just my opinion, though i suppose colouration can also be objectively measured. you could probably play sine waves through the headphone and see how closely the output signal matches a pure sine wave. if other frequencies are present then i suppose colouration is present. i haven't done this, nor do i know if anyone else has.
i have done frequency response tests with a makeshift headphone coupler and a rat shack SPL meter. i've tried it with a modified panasonic wm60a mic capsule, which has a pretty flat response (the mitey mike testing mic uses the same mic capsule). and from what i remember (i had a nice hard disk meltdown and lost all my data) my graphs looked quite similar to headroom's graphs. the grado's were elevated and peaky through the upper midrange to the treble.
but i'm no expert. this is just stuff i've picked up studying music, speakers, acoustics, and stuff...