DutchGFX you're meaning well but you're handing a rather misguided advice tbh. K240mk2 efficieny is rather mediocre and an iphone max ~1.5Vmax/16ohms will treat it only adequately at best with loud recordings with compessed dynamic range, When underpowered they also tend to have cogested presentation due to lack of speration of notes. These are studio monitor cans, They need a good amount more power than that to actually open-up. And i'd rather go for somehong truly portable and closed-back for outdooe usage. I'd do some reading about AT ATH-ES7, Sony MDR-ZX700, ATH-M50, Beyerdynamic DT770LE.
They almost the same to me from the iPhone as my E17. Just an opinion, they do not sound congested to me :) They actually get louder with my iPod than my Cowon :)