I've owned both the A900Xs and the HP100s, but not quite at the same time. The HP100s are more conventional with regard to fit and IMO more comfortable...initially. The stock earpads are a bit shallow for me, so I bought Beyer velours (the same as on the DT 770s) and they fit perfectly and don't seem to affect the sound too much, and of course they're more comfortable.
I think the HP100s are very well balanced across the spectrum and have fantastic clarity and instrument separation. Bass is clean and well controlled, but perhaps slightly north of neutral (something I prefer). I think maybe the soundstage on the A900Xs is slightly better, but I think I prefer the overall sound signature of the HP100s. The SoundMAGICS are just a touch warm, and the A900Xs are perhaps a touch bright, and maybe slightly artificial sounding. If we're splitting hairs, I think the HP100s can be a little dry at times, and maybe just slightly more sibilant than "average," but again, these are pretty fine points (these may also have been the result of the pad swapping; I didn't listen to them too long before switching). Perhaps the A900Xs do classical a little better, but I prefer the HP100s for everything else (and I can handle their tolerance for classical). They do a lot of things very well. I think they're a bit more natural sounding than the A900Xs. I think you'd do well to pick them up. For what it's worth, I smile more when I'm using the HP100s than when I had the A900Xs:). They just sound really good.
Edited by Marximus - 5/30/13 at 9:01pm