RANT ALERT!
Reviving an old issue: Golden Ear's assessment of the KEF M500. I can actually understand it. If the KEF gets too good of a seal, or if the clamp is too taut (which is what it should be imo, since they are portable headphones and should isolate, and stay on your head when walking), they do sound boomy and veiled. Also, the frequency response IMO of these is fantastic. It's almost a straight line (not with a slope of zero, more like -1/4).
I'm a little late with this (though I've had the M500 since, what now... June), but the KEF imo is a little overrated. Sure, they beat out the UE6000, Momentum and MDR 1r at home but on the go is a different battle. These are meant for portable use after all, so I think isolation should be a much greater factor.
So for me: Momentum is almost the perfect portable except that it doesn't fold and costs $350. The MDR 1R has problems with it's actual sound quality, and build quality issues. The M4U is too damn big. The UE6000 has fantastic isolation, and it folds, but it has problems with it's sound, and it is slightly larger than it has to be. The M100 has no problems really, except for it's sound signature (some may like it, some may not). The Focal Spirit One almost hits the nail with no real problems except for sound signature (not sound quality)(has great isolation, SQ, folds). ATH ESW9a lacks in isolation (not in sq, very good sq) and is flimsy. I haven't tried the P7.
The ATH WS99, FS1, UE6000, and Momentum have come the closest, however, the SQ of the WS99 and Momentum (equal) wins it out for me.
I'm not even a huge fan of the Momentum's sound sig, but I still think they rein supreme.