My [Shure SRH440](http://rads.stackoverflow.com/amzn/click/B002DP1FTU) has a very low, *poor* sound when I'm using my MP3 player ([Ipod nano](http://rads.stackoverflow.com/amzn/click/B002DP1FTU)). Since the sound gets much better when I connect it to my Receiver ([a Sony one](http://rads.stackoverflow.com/amzn/click/B002DP1FTU)) or even to the computer, I think that using a headphone amp could be good.

So, what characteristics should I look for in a headphone amplifier? I'm not sure if suggesting specific brands / models is allowed in this site, but if it is, does someone have some recomendation ???