These are amongst the best headphones available, and they are becoming harder to get now they are not being made. I sold my first pair and bought both the 400i and 560 - big mistake. They didn't have the same "you could be listening to big speakers" sound, they just sounded like good headphones. I sold them and waited until another mint pair came up - and it was quite a wait. They are not as good as the HE6 when the latter are powered by a full size speaker amp (150wpc or above into 8 ohms - if that sounds crazy, I've tried various high end amps and I can tell you that's when they come into their own, sound neutral and are probably the best cans in the world). On virtually any headphone amp, and certainly any portable device, the HE500s outclass them.
To summarise their sound, they are neutral with natural clean deep bass and plenty of detail. They also have a magic mid range and outclass the HD600/650 in this area, (which I actually found a bit shouty at times once I had got used to the HE500s). Their USP for me is their ability to sound as good as a pair of high end full range speakers, which finally enabled me to get the same satisfaction from my headphone setups as my HiFi.