Depends on your amp. That's been my observation - HE500's bass is sentimental, very weak and slightly bloated when underpowered, and very solid and impactful when given enough juice (in my case ~2.5 watts with NFB11.32).
As for impact vs DT880, assuming you got a powerful enough amp setup, it's safe to say HE500 should be more impactful.
If you are stuck with a slightly less powerful amp though, you should go with HE400, unless a powerful amp purchase is on its way.
Edited by jerg - 7/21/13 at 4:51pm