I'm not sure if people are reading this thread any longer but I felt that I needed to say more about the M500.
First, in my long and frustrating quest to find the right headphones, I have had an extended trial of (or own) VModa M80, VModa LP2, Beats Executive, Bose QC1 and Kef M500. I have also spent a fair bit of time in the Bose store listening to the QC25 and Soundtrue over ear phones.
In the real work, sound quality is subjective. We have different taste in music, different environments, different priorities, different sources and, most importantly, different hearing. In my case, I may have damaged my hearing with some gaming headphones. My hearing is certainly not as good as it was a couple of years ago.
Over Christmas I had access to all of the above sans the Kef for a full month. When comparing them, I could hear a difference but I could not definitively say that one sounded better than the other. That statement may bother some people but you must understand that in MY UNIQUE WORLD, quality was roughly equal.
All of that changed with the M500. The M500 is clearly and distinctly better. I lack the audiophile vocabulary to properly explain but it was like going from mono to stereo. It was that much better, After listening to the M500s for an hour, I put the QC1s on and thought, for the first time "oh my, that doesn't sound good at all". Terms like "richer, fuller, grander, more alive" come to mind. It wasn't louder but it seemed louder, if that makes any sense. I felt the music with the Kefs rather than just hearing the music with the rest.
I thought it was important to express just how good the Kef phones are. I absolutely hate sending them back, especially since I managed to get such a good deal (relatively) on them but the two problems are untenable. I mentioned the loose fit initially but the pounding sound when my foot strikes the ground while walking is also a big negative.
If, however, my problems are not problems for you, then I recommend the M500s without reservation.
At this time, I'm leaning toward the new Bose. They are expensive and the sound quality is not up to the Kef but they are very, very comfortable for longer sessions and that is a major concern. They also remain securely planted. I may try the ATH-M40x or ATH-M50x first but they are much heavier and probably not as comfortable for the long walks.