Summary: I recently purchased the D600's and after a few of weeks of almost non-stop listening to them, can say they are hands down the best headphones I've ever owned.... or heard.
They have the tight, low end bass response coupled with good mids and highs I've been searching for in my ideal headphone. While my AKG 701's may have a wider soundstage, and better instrument separation, they lack the bass response the new Denons deliver.
Summary: I'm in a good place. Three days ago I bought a pair of Monster Inspire headphones, which I find to be tremendous. However I let my wife try them on and I could sense a tinge of envy; I knew that I needed to have two pairs of premium cans in the house. I decided that I could solve that problem by upping the acoustic ante—by buying a pair of Denon AH-D600. I'm glad I took the plunge, because I find the AH-D600 to be equally enjoyable, compared to the inspire.
Pros: Enveloping, rich sound. Not shy on bass, but still tight. Massive, comfortable earpads. Great in-line microphone cable as well as "pure" long cable.
Cons: Much heavier and bulkier than Monster Inspire, MSRP is too high.