Well, here's one more person for the Denon's. Even though I have the D5000, the sound quality of the D2000 is very similar, it just doesn't have wood (so it won't have as much resonance) and the cable is slightly lower quality on the D2000.
The cans are incredible. Out of all my headphones, these are the only ones I am listening to now. Just be careful handling them. They are fragile and have issues with the screw on the harness. Make sure when putting them on your head you pick them up by the harness, not by the earcups.
They do not isolate well, but that is because of how comfortable they are. They don't clamp your head like a vice grip. Also, the earpad is thicker in the rear and thinner in the front to accomodate for the shape of your head, so they are super comfortable. I would say they may isolate -8 db AT MOST, probably less.
They don't need much power to drive them, but they will reveal every nuance in your music if you have a good source and a good amp. So they are pretty equipment dependant.
I don't know how some people are saying the proline can have more bass. The proline have a 40mm driver. These have a 50mm driver. The Denons have plenty of bass... bass that you can feel and hear extend down very low.
My vote goes to the Denon if you can get away with minor isolation. They are a killer pair. And for the price of the D2000, I don't know if there is a better sounding one on the market.