Your original choices of Momentum, M-100, MDR-1R, and Logitech UE6000 ($200) are all great choices for the closed, portable, over-ear category. I think Ultrasone Pro 900 is another good closed portable alternative at $300 (note: they do seem to be a love-it or hate-it type of headphones)
Comfort is very individual, so you should just make sure that whenever you end up buying from has a good return policy, especially for the Momentum & M-100 as some people commented about their fit. They do both have extremely well-rated sound, so if you found them comfortable, they are a great choice. M-100 has a v-shaped sound signature with a bass boost while the Momentums are more flat. They are the only two headphones in this price range that I have found that are made out of something other than plastic/metal if that is important to you. MDR-1R seems to be generally thought of as very comfortable.
Not considering portability, but still closed headphones at $300 brings to my mind the Mr. Speaker's Mad Dogs and the AKG K550/K551 (but the AKG does not have a bass-emphasis, so I wouldn't say they fit you well). Both are technically portable, but their larger size would give me slight pause. I personally would not pick the Pro 500 due to the price and it is a bit too bulky in my opinion for portable use. If I was going to get a "hypothetically portable" closed pair of headphones, personally I would definitely go with the Mad Dogs.
Choices that are closed, but definitely not portable include the Audio-Technica ATH-A900X and Beyerdynamic DT 770 PRO. Do not let the price make you think they are lower quality as these both have really highly regarded sound quality. Really "audiophile" sound with more bass ability than q701 from what I've read.
However, I personally think if you are going to get closed cans, you might as well get portable closed cans (because the main benefit of closed is the noise isolation & prevention of sound leakage). Also, I am not sure that you would be satisfied if you bought flat "more audiophile" sound signatures since you did not enjoy the q701, which would be a good example of more neutral audiophile sound in my opinion.
I really personally liked the M100 and that would be the headphones I recommend for a very portable, foldable, closed pair of headphones since you since you enjoyed the M50 sound. It is often described as a sonically upgraded M50 that is much more durable & better build quality. Honestly, at the $300ish price range, there are a lot of great choices! There is definitely no "best" headphones at $300. Hope this info was slightly helpful & happy headphone hunting!!
Edited by money4me247 - 3/6/13 at 12:26am