Are the headphones not satisfactory straight out of the iPod? The reason I ask is that their impedances aren't particularly low (removing any major advantage from the lower output impedance of a portable amp) and they're relatively high sensitivity (compared with other audiophile headphones) Unless they simply don't go loud enough for your tastes, there seems little point in an amp.
EDIT: Checking Headroom's graphs of impedance vs frequency for your headphones, output impedance mismatch wouldn't have any real effect on the frequency response anyway - the impedance graph for both headphones is virtually completely flat over the audible range.