That is exactly what I am hoping to find out before making my purchase, especially since it is an understatement that the D7100 would be pushing my budget. If I had the budget for a portable Summit-fi setup, I would buy a Fostex HP-P1 with Fostex TH900 headphones and call it a day. Unfortunately, I have Rodeo Drive taste and a Target budget. Since I admittedly listen mostly to compressed files on my Iphone through a FiiO E11 amp, I'm wondering if the difference between the D600 and D7100 is subtle enough that I almost wouldn't notice the difference, or if the cups and dampening actually have that much impact on these headphones. I just purchased Audio Technica ATH-PRO700MK2s. I like them for certain genres (the bass is fantastic), but they are too dark to be my go-to can (see my review under recent reviews if interested). I have read the old Denons, while having solid bass, fall short regarding bass compared to these but are much brighter. I'm also torn between the D600 and upcoming M-100 in addition to being extremely curious to audition the D400 when it comes out the end of the month- I have no idea what to expect from those based on Denon's description.
I doubt you will hear any improvement difference. If I would be you I would get D600 and stick with it at least until you will decide to upgrade ypur gear. Plus I wouldn't use $1200 headphones outside