I have heard nothing but good things about the A900 unamped. Being a 32ohm phone they are easily driven, and are said to sound best when UN amped. The basic argument is that if your not going to be using a high end source or amp, nor do you plan to in the future, these are a good set of phones, because they dont change a lot when an amp or better source is added. They sound great on anything. The A900 is said to have a lot of bass as well, they are always suggested, along with the beyer dt770 for bass adicts, which I would consider as well, in your price range (80ohm version)
From what ive heard, the open phones that AT has have a very weird sound signature. I guess to some people they are anoying to listen to, but if you do some kind of pad modification they are okay. I wouldnt trust them though, the reviews seem bad, and you would have better luck buying a grado..
Personally I am about to buy the A900s, but I keep going back and forth between thoes, and some upper end grados. I like the idea of a closed phone, but the open ones sound so much nicer.. You really need to spend a great deal of money to get a closed phone that sounds like an open one, but hey, I'm probably telling you somthing you already know.....
Edit: By the way, I dont know much about the A9x.. other than the fact that they look really sweet...they dont have the D.A.D.S. technology though, and they seem to be an older model..
My money stays with the A900, but check out the Beyerdynamic DT770 Pro 80 ohm..They had a lot of very clean punchy bass.