amps

[armandkhambatta]

um, excuse my ignorance, but what exactly do headphone amps do for cans apart from jack up the volume? and can you run a pair of top of the range akg's or sennheisers or any other reference headphone straight out of an i pod? what would the repercussions be?
I also am looking for a good open can which wont leak too much sound and will be forgiving to the few songs i have encoded in 128kbps mp3(through no fault of mine)!
i listen mainly to jazz,classical,folk,retro, and classic rock.my budget is 500$.also, my ears heat up real quick, so they have to be light and real comfy. also, i want them circumaural and not one of them flat pad on - the - ear- cans.
also, can an amp and a cable upgrade be really unforgiving to low bitrate music?
is the akg k271s a good option for the above req's(i know its sealed, but this is one can that looks so sexy im willing to compromise).
please advise, o great audio gurus!