Nobody really knows the details of how Dirac or Audyssey work because they're both proprietary solutions.
A few people have shown that Dirac seems to do full-range phase correction better than Audyssey, I don't have links right now but it's been measured. Whether or not this is a good idea, or produces audible benefit, is another story. That part has never been tested. Personally, I don't think Dirac by itself is much of an upgrade over Audyssey.
Other people might disagree, so it's very cool to have the option. In fact, now that we have both in one simple hardware platform, it will make comparison reviews much much easier! So we should actually see some pretty interesting data come out of this. We might even be able to get Amir to do a comparison review. It will be SO much easier and faster to do it with one piece of hardware rather than two separate signal chains.
For me, Dirac Live Bass Control is really the big upgrade, as it promises an MSO-like optimization performed automatically, WITH the additional capability of being able to algorithmically optimize every single speaker-subwoofer crossover independently. But so far, it doesn't seem like that's coming to Denon processors. And without that, Dirac is meh. Especially since it's very unlikely platforms that don't support DLBC will be capable of their forthcoming spatial correction.
