I have both cans and like both of them. The HD600 has a more even tempered treble where the HE-500's have a treble peak around 9-10 kHz that might bother you after extended listening or not, it's just a few db but enough to plainly hear the difference. The HE-500's have a very nice bass.
Thats interesting. I find the 500's to have a hump in their frequency, but I find it comes out more in the midrange where the vocals are. Gives them a honky quality. all the treble seems pretty wooly to me. never would have placed the spike that high. bass is killer though.