Edited by MarcadoStalker7 - 6/17/14 at 8:19pm
Given the high sensitivity and lowish impedance requirements of the 598, I think you'll be fine without an amp. Although there are several products from the Fiio lineup that are relatively inexpensive and may give your 598s a little extra sumthin' sumtin'...
I have an ipod touch 4g and a HD558 (Pretty much the same in terms of handling power). The ipod +HD558 combo fit well but if you add a lod L9 and a Fiio E11 I can hear a "not so slightly" difference in sound.