I was just browsing the net about the D2000 and HD650 and stumbled across this thread and signed up just to respond.
I really think you should get the D2000's Hybrid. I have the HD650's in the box they came in, just yesterday, ready to be shipped back for my refund. They sound exactly the same as my little PX100's that I bought several years ago for about $60. They are not worth it. I have an E9 amp powering them as well, which was an additional $110 just to get them properly powered. The D2000's can be had for ~$200-$240 if you look around and don't require a beefy or expensive amp to make it sound decent like the HD650's require.
Take a look at the Frequency response graph of the Denon's, they are so balanced across the frequencies..Compare it too a much more expensive set like the LCD-2's or HE-500's and you will see a resemblance. Some may say that FR graphs aren't very telling but when I compare the FR graphs of the HD650's and the PX100's they line up almost exactly, and they sound the same in person too. Oh yeah, and the HD650's DEATHGRIP your head and let out a ton of noise while playing.
Get the Denon's, and don't look back. They are cheaper, easier/cheaper to get sounding their best, more comfortable, and are built with better materials. I really really like Sennheiser headphones, I absolutely LOVE my PX100 (Gen. 1's), but the HD650's just aren't worth the pricetag when $60 phones can do the same on lesser equipment.
If you want to get a $2000 tube amp to make your HD650's sound better go for it, but you can just get a D2000 and be done with it for $230 and possibly an E7 amp.
I'm sure a ton of people really enjoy their HD650's but I think owning the PX100's might have ruined them for me, I feel a little cheated by Senn that they got me to buy $350 phones and an amp just to have something that sounds the same as their PX100's they used to make.
Edited by LauZaIM - 9/23/11 at 12:08am