[...] The main thing that I notice that's lacking about the D7000 compared to my Senn HD580 is the vocals. On the Senns the presence is amazing, right between your ears, and it sounds so close and personal, which is something I just don't get on the Denons, but absolutely everything else is so much better on the Denons that I can't go back to the HD580 at home. That difference could be because of the amp though, as that's the only proper amp I've tested the D7000 on.
So far I haven't done a (direct) comparison between my Denon AH-D7000 and my still rather new Sennheiser HD 650 which I've so far used with both my Grace Design m903 and Lynx Hilo.
The Denon AH-D7000 + Grace Design m903 is very good. I don't really identify with your description of Denon AH-D7000 in the above.
On the other hand I know by experience that there's a big difference between Denon AH-D7000 + Grace Design m903 vs. Denon AH-D7000 + an unsuitable amplifier.
I don't identify with this either. Of course some people/vocalists are more sibilant than others. That's nature. Listen to some people talking in your vicinity. Stone walls etc (ie. a cathedral or church) may amplify this.
I still believe that most negative descriptions of Denon AH-D7000 are based on use with amplifiers less suitable than the Grace Design m903.
As stated in my signature "I listen at (very) moderate loudness levels". This may be of importance.