This is close to my impressions. The Momentum can border on sibilance, but it's not intrinsecally sibilant as the HD25/Amperior. So you'll get sibilance with tracks that are already prone to sibilance, but in all other cases, the Momentum isn't offensive. Also, to get that tendency to sibilance you have to take it to high volumes.
The Momentum isn't exactly smooth, but not shrill, either.