At the 2012 AES convention Sean Olive and Todd Welti from Harman International presented a paper entitled "The Relationship between Perception and Measurement of Headphone Sound Quality." It caused quite a stir, with Sound And Vision declaring it "the biggest audio story of 2012. In case you missed it here's a link to the PDF slide show.
Obviously, during the past year many audiophiles have weighed in on the paper's findings, but the best overview of the study is here, written by Mr. Olive himself. What I found interesting was the in-ear measured frequency response of every headphone varied with each listener depending on the shape of their ear canals. We all have differently-shaped ear canals with their own particular resonance peaks and valleys. The "best sounding" headphone also proved to be the most consistent from listener to listener.
The study also provides hope that bass-heavy headphones are not the wave of the future. I found this paragraph particularly encouraging, "It is important for the reader not to draw generalizations from these results beyond the conditions we tested. One audio writer has already questioned whether headphone sound quality preferences of trained listeners can be extrapolated to tastes of untrained younger demographics whose apparent appetite for bass-heavy headphones might indicate otherwise. For these consumers, headphone purchases may be driven more by fashion trends and marketing B.S. (Before Science) than sound quality. While this question is the focus of future research, the preliminary data suggests that kids like headphones with accurate reproduction when given the opportunity to A/B them against colored, bass-heavy versions. There may be some clues provided by previous investigations into loudspeaker preferences of high school and college students (both Japanese and American); so far the data suggests that tastes in quality of sound reproduction tend to be universal regardless of age, listener training or culture."
I encourage you to peruse Sean Olive's Blog as well as the PDF of the slide presentation. It should give you hope that the future of headphones could be flat-frequency response, which I consider a very good thing.