What kind of amp are we talking about?
http://www.headamp.com/home_amps/lite/index.htm
http://www.ibasso.com/en/products/show.asp?ID=39
http://www.audiophilechina.com/products1.asp?S_id2=2&s_id=24
These are amps designed to drive headphones. Since most higher end headphones have high impedance they need more juice to run optimally, more than what a sound card or portable device like the iPod can provide. I have the iBasso myself. I mean, you can run headphones like the 650 straight out of an iPod, but even at a high volume they'll sound rather laid back in comparison and bass will lack impact/definition. An amp really brings them to life.
This is a plus for Bose though, since you don't need an amp to get the most out of them.