Since one is open and one mostly closed, you have such a different application. I'd try to add the HD650 instead of replacing the Denon, and yes, I realize that is not always practical but it is such an addictive sound.
I have not heard the HD650 but did have a few days with the HD700, which I LOVED. But I'd still not trade.
Yeah, I also want to keep as many as possible. Currently I have the Mad Dog 3.2 and D2000 for home usage, and at the moment I prefer D2000 has fantastic bass, soundstage, and comfort compare to Mad Dog. For mid and high I prefer the Mad Dog which for me just nice and hit the spot.
Looking the Cocobolo Rosewood Cups from Martin made me want to get one .. It is really beautiful!