Thanks for the objective comparison of the Momentum and the HD 25. Your statement above puzzles me, though, given the specs of the two headphones. My understanding was that the Momentum is a very low impedance phone (18-ohm), while the original HD 25 is in the 60 to 70 ohm range. It would seem that the Momentum should be easier to drive directly from the source. Now if you are talking strictly about the Amperior, that would make more sense as I believe Senn lowered the impedance on those.
I'm a noob, but I think it might be because SPL of Momentum is 110dB, while it's 120dB for Amperior/HD25.
Edited by eron - 10/1/12 at 8:46am