Amped impressions: I ran the M4U 2 passively out of my Nuforce Icon HD. (amp/DAC) There was a noticeable increase in spaciousness, airiness, and more bass impact/presence over passive or active running out of the MacBook Pro. It wasn't a huge difference, but it was there. A nice combination, but I still think that running passively directly out of the headphone port is the most impressed I've been with this headphone for no other reason than its ability to clean up a mediocre source. But again, they're not the final word in detail and the addition of the Nuforce did not definitively help in this regard. However, for someone looking to listen to MP3s/lossy content without the support of an external DAC I'm struggling to find a better recommendation than the M4U.
Comfort wise, I'll admit that I just can't completely get used to these. I threw my K550 on last night and was immediately struck by how weightless they seemed on my head in comparison. (along with the vast increase in soundstage) Removing the electronics from the M4U 1 should help alleviate some heft, but having the headband sit in a very small spot at the peak of my head coupled with the clamping force is really tough on me. I've gotten what seems like a pressure-induced headache (very strange... tough to explain) several times wearing these that I've never experienced with other headphones. It may just be my head and nobody else will encounter this issue, but for me this is of great concern. If there's more give to the new material used on the headband then it could make a world of difference, so I'd like to give these another chance when the design is updated.
For now, these are going back. I received my return authorization and it'll be shipped out today or tomorrow.