If you do get a portable amp it's worth tracking down one of the cables that goes from the dock connector to the amp input, rather than from the headphone output, so that you're bypassing the internal amp of the iPod. On one end it will have a plug that fits the dock connector on the bottom of the iPod and a stereo miniplug on the other.
I bought one from a seller called 'bassel-e' on ebay and it's a nicely made cable.
I bought one from a seller called 'bassel-e' on ebay and it's a nicely made cable.