[Fascination]

I'm quite new to the world of audio and I'm trying to pick out a pair of portable headphones for music on the go. After reading some material I've narrowed it down to the AKG K181DJ, Audio Technica ATH-M50, Ultrasone HFI-450, and Numark PHX. So, which one and why?
One concern I have is so called "portable amps." Its a new thing to me. In the past, I've always walked around with earbuds and my ipod. But looking at these headphones I'm reading all these comments about amps and I don't really know what to make of it. How much do I really  need one? Why do I need one? How portable really is an amp? Could I walk around with it and my ipod on my person listening to music? And suppose I were to get an amp, what should I get and how much will it cost me? 
 
Any help would appreciated. 

 

[T.R.A.N.C.E.]

you don't NEED an amp for portable headphones and IEM's. The amp just refines the overall sound. As far as portability, they are a pain to carry around, especially since the amp connects to your source (ipod) via a short interconnect, which means its hard to just pull your ipod out of your pocket and change songs. you have to pull out the amp+ipod and wires which is a pain. (not to mention the amp+ipod prolly won't fit in your pocket in the first place) 
 
Also, decent amps start at around ~$100 give or take, so you are better off spending that on getting even better phones eg, a $100 headphone +$100 amp will not sound as good as a $200 headphone connected straight to an ipod. So for now I say save yourself the hassle and cost of an amp. But if you have money to throw away, feel free to experiment with amps. The iBasso amps are a good place to start.
 
EDIT: sorry forgot to say, if you want a small, cheap portable amp then many people like the Fiio range, they are tiny and are not a hassle to carry around (but they aren't as good as the larger portable amps around like iBasso)