I just haven't got AD700 stock (or W1000 etc)
Sorry guys but I'm not really supposed to answer business related questions, but I'll give my impressions
The AD500 is a fairly flat and balanced sounding headphone, for those of you that know my tastes you'd know that I'm more of a fan of euphonic sound than technical sound. The AD300 isn't without it's flaws - the treble is fairly muffled, the bass is a little forward and the midrange isn't the most detailed but for the price they're very enjoyable (and boy are they comfy) to listen to.
On the other hand I find (and so have some people I've had audition the AD500) the AD500 to be nice and clear and detailed, bass is a little toned down, high end can be a little bright and dry so overall they're a little soulless to my ears, not much fun to listen to despite the extra detail.
The AD400 truly is a combination of both, which I find to be a little.. odd sounding, I'll have to spend more time with it to get a proper idea of where it fits so to speak.
I was pleasantly surprised by the AD300 actually
I wasn't expecting much of it but it's lots of fun.
I find the AT models to be a little hit and miss, the A700 sounds boxy and closed, the A500 is ok, but for a little more the Ultrasone HFI-500DJ1 is HEAPS better sounding (although not nearly as comfy) the A1000 sounds kind of dull, the A900 is fun but for the price I'd expect a more balanced sound etc.
The #1 best thing about AT headphones IMO is their comfort, they really do excel when it comes to this aspect.