[cyclingbobe]

The PSB and the NAD are, as others have said, comparable in sound and both have a realism that makes them superior to others at the price point including Momentum and, at a higher point, P7. It is difficult to pin down why this should be but the loudspeaker reference that Barton uses has certainly paid off. The amplifier built into the PSB M4U2 is a definite bonus and has a similar effect to a Fiio. There is a sense of acoustic and texture  which is quite striking when first heard. Either would be my go to phones. 
 
They are both very comfortable with similar ear pads but neither would win prizes for svelte appearance. The PSBs look like Robo Cop and the NADs have an odd squareness across the headband. However both are comfortable for long periods.The NADs are appreciably lighter. Both fold , in different ways, and are reasonably compact in their respective cases. The folding mechanism on the PSBs, a complex metal hinge,  seems very robust but may be vulnerable over time. It does make a much smaller shape when folded. The NAD capsules swivel flat but thats it. 
 
The noise cancelling on the PSBs certainly works but produces, to my ears, an inferior slightly nasal sound. Its very easy to compare by flicking the switch between passive, active and noise cancelling. 
 
I have not heard the PSB M4U1 s but assume they are the same as passive 2s but lighter. 
 
I am glad I have both but if forced to choose i would have the PSB for active mode and, when required, noise cancelling. For a conventional headphone the NAD is definitely the king in its price range and beyond. Between the NAD and the PSB1s I would choose the NAD as lighter and less complex in its folding mechanism. 

 

[intlsubband]

After many years of listening to headphones, mostly cheap ones, 2 years ago I started to research to buy a truly proper set of headphones. After familiarising myself head-fi and other sites, and digging as deep as I could, I decided to go for the PSB M4U2. I absolutely LOVED them. For more than a year they were my absolute favourite headphones.
 
Then, after a bit over a year, they died - one side just went completely quiet and would not make any sound. I then decided to use this as an opportunity to try a bunch of other headphones. I also updated my portable source, from an ipod classic to ibasso DX90. I even ended up getting the NAD Viso, thinking that they will sound just as good.
 
However, I couldn't get rid of the nagging feeling that even the NAD Viso's, which are reported to be just as good as the PSBs (and some even prefer the NADs), are just not in the same level of the M4U2. I knew that this might just be a bias, that I had M4U2 before I had a chance to listen to many other headphones, and that what I'm interpreting as an inferior sound is more about nostalgia than actual sound differences.
 
But the nagging feeling would not go away, so I finally got another M4U2, and I had a chance to properly compare the two back to back.
 
I'm glad to say that my memory was not playing tricks on me. To my ears, the M4U2 are definitely superior over the NADs.A part of this might be the integrated amp in the M4U2. I think that it just works perfectly well, it matches the headphones perfectly and adds a level of dynamics and realism to the sound that significantly exceeds the NADs. The bass is perhaps the best improvement - it was always a bit lacking in the NADs, and when I used the NADs with an E12, the bass was just too boomy and uncontrolled. The PSBs have wonderful bass, especially in the amped mode. Lush, warm, adds real fullness to the sound, yet clear and controlled enough to be properly detailed.

I might write a more detailed comparison between the two, but in my ears, the PSBs (especially with the amp mode) are definitely superior. I never heard the M4U1, and although I saw some contradicting reports about how they compare with M4U2 in passive mode (some say identical, some say M4U1 better), I just can't imagine the 1 competing with the sound of 2 on amped mode. The internal amp is so well implemented, sound wise, that I'm surprised I don't see more headphones with integrated amps (I was considering trying the Blue Mo-Fi's but in general the reviews of the sound are pretty mediocre).
 
I might write a more proper comparison between the M4U2 and the NADs, but for now, my "vote" clearly goes to the PSBs. Such a wonderful headphone, the complete package for a full-sized portable in my view. And the sound is actually not very far off from my main home rig (HE-500 + Corda Classic).