Let me start this review with some information about the company that made these headphones, Philips. The company that invented the CD, the first radio etc. As a Dutch guy, I'm very connected with this company. I’m studying at the Technical University of Eindhoven, where Philips has their main office and invented all their breakthroughs. This review is totally independent however. 
 
Philips only started to make high-end headphones three years ago, with the X1. It was made by some very young engineers in Belgium, and auditioned by the “golden ears” panel. 
 
There are only two acoustic engineers working at Philips at this moment, and only one industrial designer. It’s an extremely small team. And yet, they’re producing great headphones that are better than Sennheiser’s offerings. Sometimes, the best things seem to come from small teams (like the Microsoft Surface). 
 
Unboxing: In typical Philips “style” the box is very barebones black cardboard with white texts. Nothing fancy at all, no excessive use of parfume (like B&W does). The X1 had better packaging, but was also 100 dollars more expensive. I personally only care about the headphones, not the packaging. You can have stellar packaging (like Audeze), but if the headphone sounds horrible (Audeze el-8), you have only a nice box to look at. 
 
The build quality of these is pretty good, very sturdy and nicely built. In comparison with other brands in the same price range, it is way above the competition. The MDR-Z7 is already squeaking at every joint and the leather is degrading (the MDR-Z7 is a 700 dollar headphone!), I'm confident the X2 will last long.
 
The comfort of these is good, but it takes a while for the metal parts to break in. The clamping force in the beginning is a little bit high. The stretch band works very well and adjusts to your head very easily. If you have a small head, they might be too big. They seem to be developed for Dutch heads (those are pretty big). If you have a small head, the clamping force might be too low. 
 
Sound quality: not a lot of people have Fidelio products in their homes. I have the Fidelio E5 HiFi surround/stereo system. All of these products sound the same. Exactly tuned to their own house sound, as they do that using their golden ears panel, and the two sound engineers. 
 
The tuning is very neutral, no coloring in the mids and highs. The bass goes very deep, but is not exaggerated or lifted in any way. There is a sort of grain in the highs and mids. It isn’t very noticable, but in comparison with the Denon AH-MM400 they are not as clean. The quality of the bass is very addicting and in comparison with other headphones in this price range, but even above (like the EL-8), the Fidelio X2 has a better bass and lifelike sound. 
 
The midrange sounds a bit like it’s “out of phase”. This can be heard on every Fidelio product. It makes Jazz recordings very lifelike and open sounding. It sometimes doesn’t work that well with other genres, as it makes the midrange a bit “distant”. The hollowness is especially bad with poorly mastered rock music and live recordings. I can’t listen to live recordings on my Fidelio E5, as they sound very hollow, the same can sometimes be said about the Fidelio X2. It really depends on the quality of the CD. Keep in mind that a bad mastered song will sound bad on the X2. 
 
The imaging and soundstage is really something you can only get with an open headphone, and the X2 delivers in this regard. It's very good. 
 
In summary, I would recommend this headphone, as it's cheaper than it's rivals, sounds way better, is very comfortable and well-built. 

Let me start this review with my personal experience of Denon equipment. In the past few years every device and headphone of Denon disappointed me in someway. The cd and blu-ray players in failing within the first year, the headphones in sounding too bright with an overbearing bass. 
 
Nevertheless, I wanted to try out this new “Music Maniac” headphone, as it didn’t appear to be made by Fostex, but by Denon themselves, they looked great and were using a new type of driver.
 
At this moment I have the following headphones: the Fidelio x2, B&W P7, Sony MDR-10R(C), Sony MDR-Z7 and the JVC S400.
 
While unboxing the headphones it became clear that the MM400 has a striking, beautiful design. Real wooden cups, lots of metal and nice pleather. There is some plastic, but it seems to be of high quality. The sealing is of very high quality, extremely low amounts of leakage.
 
The fit is incredibly comfortable, if not the most comfortable over-ear headphone I’ve every tested. Way more comfortable in comparison with the B&W P7 and even better than the Sony MDR-1A! However, someone with very large ears might find the pads a bit too small, as they will touch the sides of your ears. I notice that the comfort is less after 2 hours straight listening.
 
In terms of sound, they do seem to replicate all the good things of the Bowers and Wilkins P7, and without the bad things of that headphone. The P7 tended to have a peak around 6kHz that made it ear piercing with some songs, the MM400 has the same very clean treble and midrange, without any harshness and with an amazing amount of detail. The bass response of the p7 tended to get a little bloaty with some songs, not with the MM400. It has a clear, tight bass that goes pretty deep.
 
The soundstage and imaging is not the best I have ever heard. However, this is to be expected from a closed can. If I compare it with the Sony's, it is a lot better. However, not as good as the Philips X2 (which is open, but doesn't sound as good in the treble and midrange).
 
In conclusion, I can say it's way better in comparison with the P7, the Fidelio X2 and the Sony MDR-Z7. It is one of the best, maybe the best headphone I have ever heard in every sonical aspect with great comfort, looks and build.