I've used many different headphones that people have tried and suggested and I find that the A40s are a great headset. Definitely recommend them. They are much better than the HPX headset in all regards. The AD700s have a little better positioning, but the A40s sound better overall, and the positioning is more than acceptable. The problem with the A40s is that they are a rip off at $200. If you are buying them with the mixamp, the effective price is roughly $120, which I feel is more appropriate. I wouldn't pay more than this for them. There are no glaring problems with this headset in any area IMO. For the price, and having used the competitors in that price range, I have to say, they are a good buy.
The AD700s are good too. There are two problems with the AD700s. First, the comfort is hit or miss. Some people like the very loose fit while others find they fit far too loose and will fall off at any turn of the head and therefore these people mod them (I did) which is easy to do. Once modded they fit ok. The second problem is that they do not have a mic. For a semi-robust solution, you need buy a mic and glue or velcro it to the outside of the headphones, which is a little "rigged" and annoying, but certainly easy enough and doable. In my personal opinion, it's not worth it to go through the trouble or rigging this headset. In the end, they will cost as much (or more) as the A40s, still not have as many features or as clean/robust/durable of a solution, and offer only a marginal improvement in positioning, at the expense of overall sound quality (IMO). Some people may actually find that the sound quality is improved with the AD700s, and that is a personal opinion, but they are close enough in quality that I'm comfortable chalking it up to just that - personal preference in sound signatures.
I know Shin hates the A40s with a passion, but really, they are a good headset in my opinion. Of the headphones/headsets I've tried, my opinion is that you'd have to invest $350 in just the headset (vs $120 with the A40s) to get up and running with a headset that is worth the improvement and trouble.
I haven't tried the A30s, but I've heard mixed reviews on them, and I feel I would be happier with the A40s. The A30s may still be worth a look if they fit your needs though.
This is just my opinion. I will say that I have bashed the A40s a bit in the past as I started to upgrade from them, but going back to them after not using them for a while, I've realized that they really are a lot better than I thought they were at the time. If Shin did the same thing, I really believe he would have the same findings.