I just returned from a week-long business trip and got right back to my HD800's and D7000's. My receiver most definitely sounds muddy compared to the Objective2 I'm borrowing, but I have an observation I can't explain that I hope someone can provide insight about.
On the O2, the Denons play louder than the Senns with the volume control fixed. On my receiver, its the other way around.
The only thing I can think of is it's a "current vs. voltage" thing.