Thanks to Jibbie for going out of his way to meet up with me today so that I could check out his Pro 900s. I just wanted to reaffirm a few points that have been made throughout this thread...
The pro 900 do indeed have very nice build quality and are indeed attractive. They are made of what seems like a very solid polymer material and feel very sturdy and adjust smoothly. They are also very comfortable in my opinion. They hold themselves on easily but do not clamp. They are indeed larger than they appear in pictures and their cups easily surrounded my ears, which I definitely prefer. The padding on the top headband seemed ample and it did not bother me at all.
As for sound signature impressions, they actually surprised me a bit. As an HD 650 owner, I expected them to be the opposite--as many have pointed out here--of my cans and be very forward, very energetic, and bright with mad scientist bass (like high-end grados without the bass). What I actually found was more of a refined and solid all-rounder, albeit with an obviously exaggerated bass curve. It had good imaging, good detail, sound stage closer to open cans, and did not seem very forward or bright (nothing like grado brightness that I don't handle very well). I left with the impression that if you Kees modded them with...hmmm..maybe 3 or 4 layers, they might be a nice, polite, and fairly balanced audiophile can.
They definitely were fun with some of my favorite downtempo electronica artists like Ulrich Schnauss and Jon Hopkins and clearly better than HD 650 in this regard as the Sennheisers are a little too smooth and warm to excel here (although I can still easily enjoy this type of music).
My brief impressions were based on listening through my laptop > foobar 2000 with Wasapi > Nuforce Icon HDP & Matrix M-stage > Denon D5000 and Ultrasone Pro 900s. Both the HDP and M-stage paired very well with the pro 900s and Jibbie and I agreed, any differences were subtle in nature.
Happy listening bassheads!