Can I buy a sound enhancer for an iPod?

I have an ipod nano, and a pare of over ear headphones. The problem is that the headphones don't have a battery by themselves like other do, so they require a lot of power from the device its listening to. So in the computer its very good quality sound. But when I listen with my iPod, it sounds terrible. Is there a battery powered sound enhancer that I can buy that plugs into my iPod that my headphones plug into, or something similar? specific items would be ideal.