Early last year Tyll did a review on the Denon lineup and generally came to the conclusion that there was a greater difference between the D2K and D5K than between the D5K and D7K.
I have not heard the D2K, but I have a coworker that has let me borrow his stock D7K on more than one occasion with the intent on using them to see if I should "upgrade to the D7K from my modded D5K. I will say that there were some genres and certain songs that I felt sounded better to my ears with the D7K, but overall, I really enjoyed the D5K much better. Where the D7K was better, it was not by that much, but when the D5K hit it right, it was more drastic of a difference. I wont call it an improvement, as it was purely my subjective opinion, but I don't like the D7K as much as I like the D5K, although this was comparing a stock D7K to Lawton tuned D5Ks with angled pads. Since these were loaners, I did not even want to try and change the ear pads on the D7K.
That said, I admit that I am not much of a sound stage fan. I don't want the music coming from behind my nose, but I'm not too particular about having a wide or narrow sound stage or even if the instruments appear to be coming from a specific location. I don't have a lot of experience with live music scenarios that I could use to reliably compare with using headphones. If this is a major factor in why the D7K is better than the D5K, than it is probably not something I would be too keyed-in on, and would not care too much in my evaluation between the two headphones.