Are these carried in any physical retail stores? I would love to see them in person for size reference to see if they'd be comfortable as portables.

Thank's Anetode! . I actually like the design, but guess the tight fit might be problematic for some.

3-4khz? Can you listen to some more tracks to confirm if its a peak or whatever it is that is going on there? I am most sensitive to that part of the spectrum.

Have you seen the measurements Tyll took?
http://www.innerfidelity.com/images/NADVISOHP50.pdf

Take a look at the uncompensated data, there's a gradual peak of sorts at that point. Looking at the distortion measurements, it doesn't seem like the peek would be too bad in actual listening (which is where things become subjective).

Edit: I'm paying attention to reviews on this headphone as well, I plan to buy them because the red version looks nice and they seem to sound good. I'm also a fan of PSB speakers but the M4U's design kind of reminded me of the Panasonic HTF600S.

3-4khz? Can you listen to some more tracks to confirm if its a peak or whatever it is that is going on there? I am most sensitive to that part of the spectrum.

After listening a bit more and trying out a frequency sweep I think it was just an artifact present on a couple CDs in a box set. That, or I was on crack. Either way the HP50s are legit.

Though I'm going to have to do something about their headband.

I'm curious as to why Tyll didn't comment on the M4U 2 any further than just stating that they "sounded good with the electronics off" in his noise cancelling headphone comparison review.

Might be a stretch, since these are the passive ones: http://stereos.about.com/od/accessoriesheadphones/ss/NAD-Viso-HP-50-Measurements_2.htm

Even though there are certainly audible differences, their similarity is uncanny. One would think the PSB's would already have piqued his interest, given how much he likes the HP50.

Not here to validate a personal purchase, but I gotta say the M4U2's are my current favorite headphones. Not as clean, detailed or linear up top as a Paradox, but the compromise in sound quality is paid back in abundance when commuting. Don't know why I've been so anally prejudiced towards NC's.. Possibly since they haven't really delivered so far. Anyway, the psb's rock in a train or a bus. Can get so much more out of the music.

Interested in the HP50 as well. Given how much the PSB's already deliver, I'm thinking these as a passive counterpart, that they are essentially, would certainly be worthwhile to any, who hasn't got experience with any of Burton's designs, to try out.