I finally got to listen to both the on-ear and over-ear version. I didn't realize I had a local "Magnolia" Best Buy and these Senns were on display (albeit with the crappiest audition amplifier and music I've ever heard, way too bright. Luckily you could bypass their system and just use your iPhone with some real tracks).
So I went back and forth for an hour. If you're looking for a punchy, bassy can, the On-ear (smaller version) has more bass and, consequently, a fatter midrange. HOWEVER, that comes at an expense. The On-Ear are far less detailed and lose soundstaging, presence and depth. I was listening to some well-recorded Broadway (Royal Shakespeare Company's full orchestral version of the Wizard of Oz) and the smaller cans just don't cut it. At first I almost thought I preferred them for the big bass and warmer sound, but there's no Sennheiser "magic" to the smaller phones. They sound great for portables, but they're definitely not a showcase for what the best Senns do. I guess they're about twice as expensive as the old PX100s were. They're certainly built much nicer and they have a slightly more sophisticated sound, but the old PX100s were REALLY portable cans -- like they just came out of the blister packaging of a twenty-dollar Sony Discman. But the Px100 where the king of cheapie phones back in the 90s. The first really nicely balanced cheap portable phone. So, in a way, I guess these new Momentum on-ears are striving for an upscale portable line and in that way they're a success. And maybe these days a hundred and twenty bucks is the price of an entry level can with a little style (I still think it's pricey). I guess it's less than a set of Beats. So as a low end can, they're nice. Really warm and punchy, but to me, they're just lacking as Senns. And I like warm and punchy.
I've owned the HD600 and HD650 (and they remain a favorite, but I'm looking for portable cans right now) and with the on-ears you don't get a sense of that Sennheiser magic, the abilityy to project realism and depth. I liked the on-ears sound at first, but the more I listened and compared, the more I realized the larger over-ear cans are the winners between the two Momentum types. You could definitely groove with the on-ears, but it's not an audiophile sound, it's just a decent headphone sound, sorta the way I feel about the much loved HD25-II which I had and quickly sold, and didn't love. The HD25s started off as a professional phone for use in stagecraft and field work and was adopted by DJs for their cool "professional" look, but they aren't an audiophile can. They're just well-made headphones, but their sound is just "ehh." Boxy and claustrophobic as you'd expect a tiny closed can to be. The 600s and 650s have a huge soundstage and they can really reproduce all the nuance of a performance, to me that's the difference. There's an open and airy sound to them (while still being "meaty" and powerful, unlike some other airy cans)
Now the good news is the large "over-ear" Momentums are quite good! At first I almost didnt' like them because I wasn't getting as much bass from them as the on-ear version, which are really warm and punchy, as I said. But wait, bassheads don't jump ship yet! And I am a basshead, I admit it. Yes, I like bass. I think you need a bass lift with headphones. Maybe it's psychological, but as nice and airy as AKGs are (which I love), with a bassier phone feel like I'm listening to a "home system" instead of cans. The Senns and their bass gives me a little of that viseral slam that you get with home speakers. Like I said, maybe it's psychological. And you also get big bass in other "real life" situations. What concert or club have you been to that doesn't hit you with bass slam? So as far as I'm concerned, there ain't nothing wrong with liking bass.
Now the "over-ear" Momentums have a SLIGHT bass lift. Much less than the "on-ears" but because of that you get a TASTE of the Senn Magic! Fantastic. I would hear it with the Wizard of Oz stage production recording (the voices of the actors were well-defined onstage and there was the reverb off the walls of the hall -- one reason I like good Broadway recordings, they're so natural), then I switched over to the Rolling Stones live album "Get Yer Ya-Yas Out" and suddenly Mick's voice was there clear and on stage with definition and a sense of ambience. In comparison, the On-ear version really suffered here. Mick was just a shadow of a presence lost in the overly bloated and congested mix. This is where the fun, but lesser-sounding on-ears really lost out. There's no Sennheiser magic in the smaller Momentums like you get with the HD600s and 650s. None of the spacial information and soundstaging that makes the 600s and 650s come alive while also being a pleasantly warm and slightly bassy phone. But the over-ears? Ah, I could sense the ghost of the HD600 just around the corner. A little taste of their greatness in a portable package!
Now the Over-Ears are NOT of the same quality as the HD600 or HD650, but you can't expect them to be. They are half the size, portable and cheaper (You can find them for around 200 bucks now)). I also wasn't that impressed with the build quality, I don't know why people would rave about it. They are about as well built as any Koss headphone from the 70s that used cost forty bucks. Not half as impressive in real life as they are in photos. Nice, but they certainly don't feel or really look "rich."
Another problem as MANY have noted it, who the hell measured people's ears and came up with the leather cup size. Many people say they don't fit, that they're too small. I agree. My problem was ONE side fit great, it slid comfortably over my left ear like a glove, but my right ear must be a few millimeters bigger and it didn't fit. Too small! Crazy, I tried stretching the cup, but didn't have any luck. Maybe with some leather softenerI c an get the right side to give a bit like a shoe and fit, but what the hell, Sennheiser, you should have been more forgiving with the size!
BUT back to sound -- despite these failings the "over-ear" momentums sound very nice FOR A PORTABLE PHONE. I much preferred them to the AKGs, Pioneers and Sonys they had to audition (Sonys are just TOO warm and bass bloated). As I said, these Over-ear Senns DO give you as "taste" of the Senn magic, they do have a slight bass lift (about half of the on-ear version, and enough to make the AKGs just sound cold and lifeless) but they are still neutral enough that they work as pseudo-audiophile cans and give you that realism that the "real" Senns deliver. So much so, that I am getting a pair for my portable phones!
I hope this helps others.
