Do I need to use an amp with the ATH-M50s?

I got the ATH-M50s a while back and I mostly use it to listen to music from my iPhone, but I'm worried that the power output might not be enough from the iPhone to power the headphones fully, like how it is with my netbook. I was wondering if it would make a difference if I were to use a portable amplifier like the FiiO E5, E6, or E7 when listening to the ATH-M50. Which is the best for a cheap price? I don't have a lot of experience with this kid of stuff. Is there any accessories that I should also get if I buy one of these portable amps?
Any input would be helpful, thanks.

FiiO E7 will work great.

Tanner - Try the headphones without the amp first - they might work just fine.