My observations;
The open AT range often have very little in common with similar numeric models from the closed range.
eg: the AD700 does not sound anything like the A700, likewise the A900 and AD900 or A1000/AD1000.
Don't assume, for example, that the A700 is "a closed AD700".
I find the difference between the A700 and A900 (closed models) to be significant enough to warrant the (relatively small) price difference. The A700 sounds "ok" the A900 sounds excellent for a closed model.
I personally prefer the AD700 (open) to both the A700 and A900 (closed) - the AD700's soundstage is wider, bass response is tighter and faster, separation is better and speed/definition is a little superior, however some people find the AD700 to be a little bright and prefer the heavier bass response of the A900.
The AD900 is a "refined" version of the AD700 (and the AD1000 is again a step up in refinement from the AD900), they have a similar overall tonal balance. If you did a very quick listen you most likely wouldn't hear a great deal of difference, but put the right music on and give them a little time and you'll notice the differences, plus going back from the likes of the AD1000 to a lower end AD model is very noticeable.
While some manufacturers tend to make specific tweaks, or exaggerate particular frequencies to emphasise a difference in models, Audio Technica tend to try to maintain similar characteristics and just make refinements/improvement to everything - keeping a particular house sound (within their parallel ranges - ie: their open models sound similar to each other and their closed models sound similar to each other, but closed and open sound quite different) - there are a few exceptions here and there where "special" models have been introduced (A1000, W1000 etc, where the A900 and A1000 have little in common).
Basically if you're a fan of the AD700, you'd probably appreciate the AD1000.
However, if you like the A700 it doesn't mean you'll like the AD700 or AD900/AD1000 because the open models sound quite different to the closed models (again, with a couple of exceptions here and there).
In short - if you want closed, stretch the budget a little and get the A900 over the A700 - it IS worth the difference.
If you want open, the AD700 is nice, but the AD900 is better
I personally really like the AD900 (open), I don't mind the A900 (closed) but given the choice I'd take the open model.
I really wish Audio Technica would use different model numbers to avoid the confusion between open and closed models
I hope people understand all this