I'll jump in on this one - looks like fun!
Fortunately there is a wide range of headphone brands and models out there allowing us the opportunity to find the best phone for our own taste and price range.
I have owned both the D2000s and HD650s. I bought the D2000s as an upgrade from my Grado SR60s and intended to use them at work.
I listened to a wide range of music with them and from my point of view - I just didn't like them. The lows didn't really seem that bloated to me although the extension caught me as being a little over the top. I could live with it though as it did make some styles of music fun but others it became almost annoying.
For me the mids were really funky, very hollow sounding, almost as if my head was in a bucket - whatever you want to call i didn't find it at all natural. Then there was the highs which were the worst part for me. I found the highs to down right offensive. Some may call them fast, or detailed and I agree I did hear detail that I wasn't hearing in my Grados but the sibilance was constantly making me turn the music down to protect my hearing - it was just over the top harsh. I ended up returning my D2000s - I actually preferred my SR60s to the Denons buy a large margin. I struggle to see how these headphones could be so highly thought of. Yes they seem very well made, the metal and leather on the D2000s seems like a nicer build than the HD650 plastic - but at $350 I think they are maybe about $250 over priced for my ears and even then I would still take the $80 Grados.
The HD650s for me on the other hand seem much more accurate in their sonic character and the detail in the highs and mids is spectacular. The lows could maybe be a little punchier for my tastes but other than that I truly am enjoying them. To me - the HD650s are in a totally different class than the Denons with respect to sound quality.
As for measurements - take a look at the HIFIMAN HE-500s and compare them to the Denon D2000s. They are almost the same frequency response curves. Totally different technology, one is open, the other closed, one is a planar magnetic, the other dynamic, but very similar sonic frequency responces. Presonally I love the HE-500s and would love to own a pair but I truly find the D2000s to be downright offensive sounding. Measurments are one thing, what sounds the best is totally subjective and might be something completely different.
Isn't it great that there is more than one model of headphones on the market?