Wish I could comment on that, but I've only heard mine.
But I have learned to discern certain FR over the years, or to know emphasis in areas. I'm no expert, but the HD800's to me are as flat as you can get. The only area where they cheat - and most quality headphones do, is a dip at the 2-3k mark. But this is where we are most sensitive according the earologists (yes just made that word up coz I couldn't be ***** to wiki it.) and we have a natural dip there. The K701 by contrast have this area slightly emphasized and just above the line. If the Senn's were flatter in this area, we wouldn't make such a deal of the 6-7k peak imo.
I don't envy headphone manufacturers trying to decide how to "tune" the frequency response. Between the variations in amplifiers on the market and the target demographic, it's impossible for a headphone to please everyone.
Consider the Equal Loudness Contour (ELC) and sensorineural hearing loss of individuals. I have often wondered if engineers design certain headphones to sound ideal for a specific demographic using as a model the typical hearing loss curve of the target age group and compensation for the ELC based on typical volume that age group might most often use.
Oh, and LugBug, since you opened the door - is Otolaryngologist the word you were looking for? I had never heard this word before a couple of months ago but now its part of my lexicon. It's fun to type and even funner to say!