I think we're mixing around transparent with natural. If you look at the measurements on the HE-500 (not to hate on it, as it IS a great headphone), it's a bit hot in the treble region. That is, however, the trend among cans nowadays, so it's understandable. I happen to find hot treble intolerable, while many find hot treble to be exciting or even revealing, as a great deal of lower level detail is rendered in the higher treble ranges anyway.
Well, I never said the HE-500 was transparent or neutral - or did I? Yes the HE-500 is a little hot, but it can be tamed by something like a balanced M^3 or a βeta-22.