With headphone origins in the mid to late 60's and more seriously in the early 70's, most of the cans my friends and I listened to (and could afford going to school) were very "n" shaped in their frequency response. As I recall, all or virtually all we had were closed. We used our headphones to listen to music--most critically from decent turntables and reel to reel tape @ 7 1/2 i.p.s.. Personal computers did not exist; the Walkman, beta and VHS had not yet hit the market, and TV was over the air and in mono. What has not changed since then is that the source of the music and all the stuff that got it to your headphones (or AR 2a's) was important to how it sounded. To determine if a can was better than the average we listened for two things--1) whether it had increased clarity, and 2) whether the high and/or low ends extended and/or was an improvement over what we already had.
Fast forward many decades to now--I'm looking not only for a full (and at least relatively symmetrical) frequency response 20 to 20 and clarity, but in addition a good sound stage, sub bass (but not to excess) and musicality (the most subjective of all criteria).
Edited by KG Jag - 3/29/12 at 3:05pm