You're in luck, I also have both the DT770's on hand as well as the DT990's. I just spent the last 20 minutes scrutinizing all 3, and I would say the 990's are a teensy bit wider than my 6XX, but the 770 is a bit narrower than my modded 6XX. (No EQ or Crossfeed)

I would add though, that in my possibly biased opinion, my 6XX's now respond well to crossfeed in the L and R channels. I've sometimes played sound in them thinking my monitor speakers were on instead!