The DX100 will be MORE than enough to drive the HE-500. Hell, they sound almost good as my UM Miracles out of my HM-601 (as good, actually. HE-500 has a wider soundstage, doesn't have monotone bass. UM Miracle has a more accurate, cohesive soundstage. I found the HE-500s more comfortable and had less microphonics). I can't even imagine how good they will be when I get my own DX100.
The only reason I have customs is that the HE-500s are NOT portable. They're huge, 500 grams, open, leak sound, and have very very limited isolation.
For homee use in a quiet place, the HE-500 is an absolutely amazing can.