Careful for some of the AKG Q701s. Some are made in China, some are made in Austria. Since I have not heard the China ones, I'm not sure how good they are. The ones made in Austria are amazing. If you are going to pick up a pair, I would suggest finding a pair that is made in Austria, just to be safe. It should be written ("Made in Austria") on the outside of the earcup under the "Q" which is in the centre of the headphone. with the "Reference Headphones Q701" above the "Q".
No I have not heard the ad700x. To be honest AT is known for their headphone almost always if not always being bright. I try and avoid them a little, (unless one pops up at a bargain price so I can try them) but to my understanding, the q701is undesputedly soundstage.king under $400. To be honest, it has such a wide soundstage it feel un-natural to some, artificial. If you're ok with that, which most peole are (including me) then there is no better match. But i still do agree the soundstage is unnaturally wide, and will probably be the widest that I will ever hear.
It isn't, IMO, the pure "distance" feeling you get, but I feel a good soundstage when coupled with a certain airiness combines for a more realistic feeling. Airiness is something that opens offer with much less effort than closed-back. Effort, as we all know, often translates to price-tag in the audio world.
As to your question ggilman, I haven't tried the Q701's/K701's but the AKG opens and semi-opens that I have heard bring a lot of air into the mix. The cheaper AKG K141 MKII has a great soundstage for example, as do the K240's. Both headphones that scale up exceedingly well with a good source. I imagine the Q/K701's too scale up as well.
Don't forget that the source is a very important factor. My DX50 died a short while back and I'm to send it for repairs once I finish moving house. As a replacement I got a FiiO E18 Kunlun to connect to my Android device, and the pairing between it and my current "heavy-use" headphones (SoundMAGIC HP100's) is heavenly.
The point I'm trying to make here, longwinded though it may be, is pair your headphones with your source in the best way you can. I think it better to spend $150 on a source to be paired with $100 cans than just buying $250 headphones.
That being said, for a fair while I was satisfied with AKG K141 MKII's driven from a 2nd gen iPod nano. Those or the K240's I think might be good starting points if the Q/K701's are too pricey., and then as time progresses you can upgrade your source. This is all very general because your profile has no information so I don't know where you stand on DAC\Amp inventory.