Let me be right up front and say that my employer (and therefore I) can derive financial benefit from what I'm about to write. On the other hand, please know that I have been an AKG fan and user for over 25 years and I honestly believe that the AKG K 501 is a class-leading headphone.
The K 501 has audiophile sound--smooth mids, clean highs, accurate low end all very well integrated. There are no annoying colorations in any part of the audio spectrum with the K 501. The K 501 is very comfortable too--it has a genuine leather headband that takes the weight of the heaphone off my ears. The gimbal-suspended ear cups perfectly encircle my ears and I can wear them for hours without feeling like I've been wearing a football helmet. They aren't the most efficient 'phones on the market, but I can plug them into my computer or minidisc player and have enough level to please most anyone except rap/metal fans. As far as side-by-side comparisons I've done, the K 501 is far more detailed than the Sony MDR-7506 and V700DJ phones and has a better balance than the Sennheiser models. I know that good sound is very subjective, but for the music I listen to, what I like to hear, how I use the headphone and how comfortable it is, the K 501 is my pick.