If I were you I'd send a mail to the company, there's no way of knowing for sure as it's indeed not listed. And it's battery powered so that makes me doubt if it'll be able to deliver.
The HE-500 needs amplification but any reasonably powered desktop amp should be able to do it. Nothing against speaker amps but hooking them up to all kinds of headphones looks like the flavor of the month to me, with the exception of (maybe) the HE-6 I see more potential disadvantages than advantages vs headphone amps. I own the HE-6 and have a HE-500 on loan (and like it so much that it'll become my next purchase). What I can tell you conclusively is that, while the 500 needs some juice to perform, it's MUCH, MUCH easier to drive than the 6 and when I hook up the 500 to my EpiphanyAcoustics o2-dac/amp they sound great without hitting extra gain (I listen to classical so typical recording volume is low but dynamic range is huge). As comparison point I have my Violectric V200 which outputs 2.7 W at 50 Ohm (and which I also use to drive the HE-6, something that my O2 definitely can't pull off). But no experience with the stepdance so really can't tell.