Thanks Jude for the heads up on the Momentum. Got it yesterday and been using it for about 6 hours. It ran overnight at modest levels with the Grace M903 at a level of 63.
First off, the apple did not fall far from the tree and the Momentum sounds very much like the HD800 for my jazz, classical and classic rock pieces. It played Bach: Toccata & Fugue In D Minor, BWV 565 (Telarc) with command. Not LCD-3 command but far more than my Philips Fidelio L1 and Grado SR325. I would even say the bass was better than my Grado GS1000i. That's saying a lot!
Soundstage wise, well, it's not the HD800. It's very good. Excellent in fact with my iPhone 4S. I will no longer use the Philips Fidelio as the Momentum is not only lighter but it totally disappears after some time and all I get is music. I hardly feel that it's there. There is no sweat after an hour, two hours and beyond. I wear a size 8 cap and these new Sennheisers are simply the right fit on my head. I wish my LCD-3 were this comfortable.
With the Grace M903 with which I did most of my listening, the Momentum sings like my top-line phones. With the Cary 300SEI, the sound is even sweeter.
A great phone, so far, for classical, jazz and classic rock.