I would guess it's a gradual hump as it causes one bass note to blur with the next one causing a dulling effect. It can be pleasant but I find it robs too much from attack and transients (in bass area).
Try this track, http://www.youtube.com/watch?v=3zIzy3laQck. I find it easier to pick out the individual fingers plucking of the banjo with the LCD2 while they sort of blur with the HE500.
I'm not sure what causes it... but... it sounds like a dull, slower, slightly muffled sound when compared to any of my other phones, which sound faster with greater PRAT and attack. The leading edges of the notes are dulled, rather than sharp.
While there is a mid-bass hump, that is not especially objectionable, though it may be contributing to the "thick, syrupy" sound I describe, in the manner you mention.
And... your brain does adjust, somewhat, to the duller sound, so it is more difficult to identify, except when A:B'd with my other phones - as Sharose notes.... though... I still notice it, because I'm sensitive to it.
My only question is... does any good headphone amp (which drives multiple phones) clear them up? I assume no, since Shahrose has tried it with a pretty powerful amp... so, long... as it has the high current output required by the HE500's (in contrast to high voltage output).
Edited by Gradofan2 - 8/5/11 at 12:27pm