Man, I've been listening to the AD900X for a couple of days now. I actually thought they sounded great on open box but heard some graininess in some tunes. It's 20 hours later and I actually don't heard the graininess on the same tunes. It has refined nicely already. I don't really hear any flaws so if it still gets better than this, I'll be smitten.
The sound of this open headphone is stunning. The airiness is the best I've heard. The music surrounds your head, it's fantastic. I have the HE500/400 sitting here and the Fidelio X1 too and I think these have the best stage of all of them.
I think the best feature about this headphone, besides the stage is that all the ranges are even. The bass is nice and tight and never spoils any other frequency. Thankfully the bass is present since everything else I own would be considered to be bass enhanced compared to these. I love the bass because it's so well controlled.
Vocals sound so good too. It is forward but not even close to being too forward. Positioned correctly in my book. I feel like I am listening to a live recording. Amazing.
Treble. This part I was scared of since almost all AT earphones get too sibilant for me. For instance, the new CKS1000 and CKN50 both I've heard still get somewhat strident but is much better than the older models and I can handle it. More so on the CKS1000. Well, these AD900X's don't even come close to sibilance but still keep that AT treble flowing beautifully.
I've heard a lot of gear since I've been on Head-Fi and a piece of gear like this comes my way every once in awhile that just blows me away thankfully to some close friends I've met on here to recommend them to me. This headphone is a pure bargain for the sound you get.