I might consider getting the AKG K550 with an amp if it's worth the extra money - would there be a significant difference between the quality of an amped AKG K550 and, let's say, a grado/alessandro, Ultrasone HiFi 780 or KRK KNS-6400? If so, how much would you recommend spending on an amp?
The Grados/Alessandros are open cans with a sound signature very different from the K 550 (or open Q 701 for that matter). If you go amped the comparison cans move from those listed above to the better Ultrasone Pro 750 and KRK KNS 8400.
I only have a desktop amp and don't have my main music in the form of computer files. My HiFiMan EF2A will probably work well for you, as it has a good built in dac and usb connection. However, what you probably need is combination destop and portable amp/dac combination. Fiio (and probably others) offer such a product in a fairly affordable range.