HD25 are true over-ears, and they look pretty silly when worn around town. Also they have the least soundstage from any "quality" headphones I've tried. Surprisingly flat-ish frequency response, though.
Momentums don't have a 300 dollar sound, but that is not what they are about. They are good looking style-phones, that are made to be used out & about. To my ears they need amping much more than my M100's for an example. Straight from my iPhone 5S or iTouch they sound pretty dull and loose, but with my Fiio E7 they sound much, much better. Nice soundstage and thump in the bass. Mids are quite nice really. Hell, with amping they could be called the "portable HD650". There are lots of similarities.
Momentums have some extra low-end, but it is really needed when sitting on a bus or walking in traffic. A flat bass would get simply drowned out completely. This is why my Focal Spirit Pro's need a hint of boost down low. They don't at home, though.
So, in conclusion, this is a ridiculous topic.
Edited by sikki-six - 11/25/15 at 9:57am