I think the best approach is to buy the HD800s before selling the D7000s. It's really hard to know how much you're going to like a headphone until you have some serious time to spend with it. If the HD800s are all you feel the need, then you can sell the D7000s without any buyer's remorse.
I agree with others that it's very handy to have both an open and closed (or semi-closed) headphone at your disposal. Sometimes you don't want to disturb others, or want some isolation (however less than ideal the D7000s' isolation might be).
Here are some points about my own experience with the HD800s and D7000s:
- I find that the HD800s on a good recording are the best sounding of all my headphones, clear and natural. But on a few recordings, I find them unlistenable, with buzzsaw-like treble. I know some people find the D7000s' treble a tad harsh, but it doesn't bother me. So far the HD800s are more troublesome in that regard, although like I said, just on a few recordings. As a point of comparison, my SRH940s are brighter and harsher than both on far more recordings, and I have to EQ down the treble on those just to generally listen to them.
- The bass on the HD800s is beautiful but not strong. So if I feel like thumping along to rap or electronic or some deep John Bonham drums, the D7000s are infinitely better. The D7000s are more fun.
- The D7000s are more euphonic in the midrange. There's a rounded, warm sweetness that I have difficulty describing but I find addictive. In contrast, the HD800s sound effortless and detailed, but don't have that sweetness.
- The HD800s have a more open sound and better soundstage. Not a surprise, I'm sure!
- The HD800s are incredibly comfortable to wear. The D7000s are above average for comfort, but the HD800s are considerably better. The cups are huge, the clamping force is low, and the weight is distributed so nicely around the ear. The D7000s in contrast are a little uneven in how they distribute the weight, pressing a bit more in front of my ear.
Thanks for the response and advice Dan. I probably will buy the 800's and live with them both for a while.
FWIW, I don't consider the D7000's "closed." They may appear to be closed, but they aren't.