okay, so now I have tried the M100, Momentum, AKG K550, and MDR-1R very extensively (2wks+ each). I personally thought all the headphones sounded really nice. of course, each one had their own particular sound signature emphasis but after I listened to one particular headphone for 1 whole week, I felt like my ears adjusted to the sound signature. The biggest jump would be from the M100 (super bass emphasis) to the AKG K550 (I felt that they were very neutral & treble-emphasized). However, I really loved both the AKG K550 and M100, so I guess I am not very picky with my sound signatures (I appreciate everything as long as it is well-done: flat, v-shaped, and bassy).
Seriously, the mdr-1r are hands down the most comfortable pair of headphones I've tried by a huge margin for me. I'm in med school, so I study for really long periods of time (4-8hrs+) w/ headphones and from my personal experience comfort goes like this: MDR-1R > AKG K550 > Momentum > M100. The M100 has shallow earcup depth that really bothers me personally. The Momentum are comfortable for on-ears, but I definitely prefer straight over-ears. The AKG K550 is never uncomfortable but they were a bit too big for me, so a bit loose. Also, I felt that the AKG K550's sound signature did actually change if I shifted the headphone position (most noticeable w/ bass). The MDR-1R fit me really snuggly (headband, earpads, and everything!)!!! It was really like I wasn't wearing headphones at all.
Keep in mind comfort is very personal, but if you need a pair of headphones you can just slap on & forget for long marathon listening periods, I really think that the MDR-1Rs are a great starting choice!
In terms of sound quality, I actually really really loved the well-done bass boost of the M100 (seems really textured & detailed bass to me). However, I also really loved AKG K550 for it's very sharp/detailed treble. I also thought that the AKG K550 had really nice bass as there seemed to be great bass extension into the lower frequencies, but the AKG bass is definitely not as emphasized compared to all the other headphones. Momentum didn't really strike me as anything special, but it was really well-balanced w/ a slight bass boost (more bass than the akg, but i thought that the k550 bass extended into lower frequencies or at least it was easier to notice the lower frequency bass on the k550).
mdr-1r seems the most uniquely colored pair to me. i think i might actually prefer the sound signatures of the other headphones, but the mdr-1rs definitely also sound good. they are just different. i think it might be because they are more "mid-centric" and I am more used to v-shaped sound signatures. also, they are the ones I had the least amount of time with so far, but they are definitely also solid choice!!!
I really think that people would benefit from doing a test run between all these headphones!! AKG K550 & Momentum are available from Best Buy with a 15-return policy. MDR-1R from Sony Store or amazon & M100 from amazon/v-moda. I actually also borrowed my friends Beats Studio & Bose QC15 to see if I can tell why other headphones are more recommended by audiophiles. It is really surprising how subtle/obvious some of the differences people say about these headphones are and also surprising how I definitely disagree with a lot of other owners' opinions on the SQ. You really need to try it for yourself to see!!!