IMO, if you are going to use the SE530 on the street, go ampless. I have a Hornet M, ALO LOD, iPod classic and Shure E500 (same as the SE530). When I'm out for a walk, portable amps are more of bother than they are worth. I admit that a portable improves SQ and it removes the very slight hiss that you hear with the 530 plugged straight into the iPod, however, IMO when you are out and about the SQ improvement is not worth the inconvenience of having to carry another piece of equipment. I do use the Hornet with my full size cans when I listen at home but away from my home amp. I use to own the original SR71 but I like the Hornet better because it has the gain switch and it has more gain for low sensitivity HP like my full size K340s.
My advice, for portable use, go ampless. If you must get an amp, get the new SR71A because its probably the best SQ and it will have the gain switch for greater versatility.