I've been listening to my iPod with my Sennheiser HD 518 Headphones with an adapter and am considering purchasing a portable amp for it. I was just wondering if a portable amp would be worth buying and if so which one would you suggest for me.    I listen to mainly Hard Rock, Heavy Metal...

The adapter that came with my Sennheiser HD518 has stopped working, and since I only listen through my computer and iPod, they are rendered useless. What is a good replacement? (Completely new to the headphone world, sorry for my stupidity.)   Thanks in advance!