I've heard the AKG K501, and other than it being a bit light on the bass, it sounded good. Be advised, however, that you'll need quite a beefy headphone amp in order to adequately drive that 'phone - it's even less efficient and less sensitive than the Sennheiser HD 600.
I haven't heard the AKG K271S Studio headphone. But from what I've read on Headroom's site, the 271 is a HUGE improvement over its K270S predecessor. In fact, they prefer this 'phone over the (in their opinion) somewhat overwarm Beyerdynamic DT 250-80 (which is still good, in their judgment) for studio monitoring use. The K271S is easily powered by an unamped portable but will sound better with an amp.
Hope this helps.