If it's a later model - check out the Equaliser app from the i-tunes store. It's pretty mature now, and a better way of shaping the sound than having to rely on a portable amp. If you want something even easier - the Accudio Pro app is pretty good too. The actual software equaliser included on the ipod/touch/iphone etc is rubbish. The add-on apps though are actually pretty good. With a bit of practise, it's amazing how well you can shape the sound without having to buy a warm amp with hardware EQ (ie bass or treble boost).
Just another option - and this one won't break the bank
It's an app by Denon. You can create points on the wave of the song to deepen the song, ie, make your own EQ or use their presets, which can be tweaked to your liking. It's also a decent music app.