New(ish) measurements of Clip+, iPod Touch 4G and Altmann Tera-Player : http://www.tera-player.com/Tera-Player_RMAA.html
What is your source on this? Frequency response is tailored in the studio from the microphone to the mixing console, it is then tailored again by EQ, so the frequency response of a headphone or PMP is not paramount at all, it can even be fixed with an equalizer to a greater extent.
Settling time, impulse response, square wave performance, phase error, glitch etc. can not be fixed with any kind of equalizer and thus they are of more paramount importance to the final sound.
In conclusion, frequency response and cumulative spectral decay are of lesser importance in the total picture, you shouldn't focus on a tree and claim it's a forest unless you have evidence.
There are some instances where frequency response is paramount or desired, and in those instances an Etymotic ER-4B will do the trick, I don't see them being used in studios very much however.
Edited by Theta Alpha 1 - 5/28/13 at 7:30pm