I was finally able to satisfy my curiosity to hear the MDR-1R by making a special trip to the Sony store in downtown Philly this morning. These are general impressions from less than an hour of listening, so please take with the obligatory "grain of salt". Parking was $16, but that is cheaper than return shipping. Comparing to M-100 and what I remember from my time with the M4U2, the forward mids of the 1R jumped out right away. While I thought detail was good, I was thinking while listening that, if vocals are this much out front, they need even MORE detail to take advantage of their presence. I don't listen with lossless or FLAC files. I use Apple Plus (256K), so perhaps lossless files would provide more detail, but I prefer the signature of my M-100 for most music and am even willing to acknowledge that is may be due to laid-back mids on M-100 being more forgiving. Highs extend further with the 1R, but I still find the M-100 smoother and more enjoyable with more prominent bass that I prefer. Bass presence is there with 1R, but coming from M-100, I was missing it with some tracks. I would equate the difference in midrange presence between M-100 and 1R to equal the difference in bass between 1R and M-100.
I also feel the 1R has as much extension in the high end compared to the M4U2, but the M4U2 provides a smoother, more natural presentation overall than the 1R. However, I consider the highs of the 1R superior to both Momentum and Amperior which I found to be somewhat harsh. 1R is also more detailed than Amperior; comparable to Momentum.
All five of these have a strong bass presence to varying degrees.
For what it's worth, here is my attempt at objectivity, ranking for overall listening enjoyment IMO with all genres:
1. M4U2
2. M-100
3. MDR-1R
4. Momentum
5. Amperior
Here is my ranking based on personal taste:
1. M-100
2. M4U2
3. MDR-1R
4. Amperior
5. Momentum
While I have no reason to buy my third choice, I feel MDR-1R is a worthy entry in this category and a solid performer, especially if you are a midhead
looking for something closed and portable. They also take the #1 spot for comfort as they are spacious and light with a gentle clamping force, yet they don't fly off if I shake my head.
EDIT: While at the Sony store, I also A/B'd the XB500 (which is on clearance for $50) and the MDR-V6. As impressive as the bass was on the XB500, there is NO WAY I could enjoy listening to music through them. The veiled highs are a dealbreaker and I wouldn't even pay $50 for them. On the other hand, I was impressed with the MDR-V6 for the price. While it didn't match my preferred sound signature as I am sensitive to bright highs, the highs extend MUCH further than the MDR-1R with good detail and no significant sibilance. Of course, mids are more forward, but with respectable detail. They are much weaker on the low end compared to the MDR-1R, so if you tried the MDR-1R and thought the highs didn't extend far enough and you are not a basshead, save $200 and check out the MDR-V6.