I'm very happy with the way that the Headstage Arrow is driving my HD650, and I'm waiting on a D4 to arrive from elsewhere - should be here Monday / Tuesday next week, so can't post a comparison at the moment :)
I doubt you'll have any distortion issues at any volume with the Arrow. I'm using gain II (it goes I / II / III), and don't think I ever go above 30-40% on the volume control. Can't believe how powerful such a thin amp is!
It's definitely a lot better at driving my HD650 then than my old iBasso D2 Boa, but then as the D4 the potential of running from a 9V battery (I think the Boa only had 3.6V?) that could make a lot of difference. I've got the Topkit ordered too - not 100% sure of what it does, but probably worth a search if you haven't looked already :)
Edited by Bencrest - 7/1/10 at 1:53am