Davie,
I haven't heard any of the AT headphones. I recently purchased a used pair of W100's that will hopefully arrive next week. I did a lot of research and asked a lot of questions before I purchased so I'll try to share some of my findings.
First of the A500, A700, A900, A1000 and W1000 are the newest offering from AT. The A100Ti, A55, A9X and W100 are previous versions. From what I can gather generally the AT sound is livlier than the Senn 580/600's. Somewhat brighter (but not as bright as Grado's), decent midrange that can sometimes lean towards being a bit recessed though I've heard this is not the case in the A1000 and W1000. The newer line also incorporate D.A.D.S. which as far as I can tell is a venting system that allows for better bass extension and quality. The AT's are said to have some really good bass in most of their headphones even the older non D.A.D.S. offerings.
Of notable exception seems to be the W100. While from what I understand it still has the AT sound but it's midrange is said to be a bit more forward giving it a more lush quality. I've heard conflicting things on the high end, some say they're a bit bright others say they're not. The low end supposedly has a mild bump in the upper registers and overall lacks the ultimate extension. This headphone is often described as drawing you into the music and having an almost tubelike quality.
All of these AT headphones are closed backed. Their open air offerings have not been popular in these parts and the one person I know that bought a pair was not impressed. To explain a bit further. Open backed headphones generally give an airy wider presentation, with greater sense of space. Closed on the other hand have just that, a closer and (to me) more personal presentation. Done badly there is little soundstage to speak of other than blobs of sound on either side of your head and in the middle. Done right and the presentation can approach the level of open headphones. FWIW, I've heard the AT's presentations are some of the best of closed headphones.
Now if you were thinking in terms of the HD600 then you would probably want to look at the A1000 or W1000. The sound is said to be very similar if not indistinguisable between the two so it would come down to a matter looks. Personally given a choice between the two I would go for the W1000 as I suspect it may have a slight edge in sound and they're just so damn sexy.
Again, remember I have not heard any of these. My comments are based solely on my research before purchasing the W100's. If I've made any errors I hope they will be corrected.
Hope it helps,
Phil