I exchanged my 550's for Ultrasone Pro900's and where I'm not completely satisfied with them...They're better than the AKG...for me. The lack of richness and low end combined with the poor fit got to me after a couple of weeks. I also like the accessories that Ultrasone include with their proline. Case-Cables-Pads. The fit is great too. I can see with certain types of music they could be fatiquing. It all depends on your preferences.
I'm learning that headphones are like women... You'll try many before settling down but even then, you keep looking around thinking there might be something better out there.