Advice on Portable Amp needed.

[heatblast360]

Well I have been using a pair of Audio Techina Headphones XS-7 for about 3months and I usually play songs from my iPhone. The ohm impedance of those headphones are roughly around 40ohms and most of my songs are about 320kbps. i feel that the bass of the headphones are weak and not strong enough, the vocals are also not quite thr. I was wondering if I require an amp. I am very in favour of the Fiio E5 amp. Fiio E5 amp is pretty cheap and there are many good reviews about it. So do I require an amp? maybe you guys can provide some suggestion of a cheap and good portable amp.