What does the AD have to do with the comments that you made about the MD being congested? Nothing that I can tell. The AD takes a good product and improves on it. The MD has, since it's first iteration been a very "open" and "uncongested" sounding hp. I feel that I can make this assertion because I've owned them all since the beginning and have also heard the AD and know exactly where it stands with the rest.
These are fine hp's but certainly have their flaws as all hp's do. It's just a matter of what compromises that you're willing to live with and what you're willing able to pay to get what you want out of the experience. But of the weaknesses that either of these hp's have, "congested" sounding is not one of them.
I've owned more than a half dozen closed hp's and I've certainly heard many that could be characterized as congested...closed in or whatever semantics you'd like to use to describe that phenomenon. The m50 suffered from it as did the dt770, d2k, d5k and d7k.
But, in my estimation, the MD is actually the most "uncongested" closed hp that I've heard. It competes with decent open hp's...the hd650 comes to mind. I feel this is one reason that has lead to the MD popularity here.