I haven't been watching the headphone market right now. From what I can gather from a quick research, are the new line of Sennheiser headphones (598, 800 etc...) a departure from their trademark sound signature?
Another thing has to do with the comfort, the felt pads on the HD555 and 600 tend to absorb sweat and becomes uncomfortable after a while. Also, Sennheiser asks you to replace the pads every 2 years. What kind of headphones are the best in the sweat department?