Some quick and early impressions here. I'm comparing these mostly to my other closed set of headphones, the NAD Viso HP50.
 
You get a *little* more clarity and detail, bigger and more impactful bass with more extension, much better instrument separation, and better-looking cans. 
 
What do you lose? Well, if there's one downfall for the H6s, it's that vocals are thin and a little recessed. That's really the only major con for the sound in comparison to the HP50s.
 
I'd consider the HP50s to be slightly more neutral. But the weakness of the HP50s is that the bass doesn't extend right down, nor does it have much impact. Whereas the H6s are one of those cans where the extension of the bass right down can really make you *feel* it. And the strength/weakness of the HP50 is that there's a little less impact, so they're less fatiguing than the H6.  
 
Perhaps the HP50s also have a tad larger soundstage. A few more moments where you think there's someone standing behind you in the room, but both possess a reasonable soundstage for a *closed* headphone. But I wouldn't trade the soundstage given that the HP50s are also a touch muddier and a touch more congested (as I mentioned regarding detail and clarity and instrument separation).
 
Sound leakage is a little worse on the H6s, but neither are terrible. I have a super-quiet office, and I can still get to medium volumes on both without bugging other people.
 
Now, to put thing in perspective, my Fidelio X2s still best the H6. Yes, the X2s are a little grainier in the mids/trebles (the H6s can definitely be described as 'smooth'), but the X2s still have more clarity, detail, bass impact, better vocals, etc. (They do have the advantage of being open-backed headphones, after all.) I feel like the H6 from both a sound and quality perspective, sit between the X2s and HP50s.
 
So, what I'm telling you is that the H6 are very, very good for a CLOSED headphone at this price without performing any miracles. They're my favourite closed headphone. Are they better on a dollar-for-dollar basis than the HP50s? No. Both are priced about right regarding sound quality, I think. It's a matter of how much you wish to spend.
 
However, do also take into account that the H6s are better built and better looking than the HP50s. 

I occasionally want to use my Dragonfly Red DAC with my desktop computer. I've never had issues with the Dragonfly from my laptop ports or my phone's micro-USB port. However, when connected on any USB port of my desktop, I get a loud hizzing/buzzing noise that ruins the sound of the DAC.
 
So, I bought the Schiit Wyrd. It fixed the problem perfectly. Zero hissing/buzzing whatsoever. Clean, clear sound. 
 
I'm not going to tell you that it magically improves the sound of the DAC. Nor can I tell you whether or not other cheaper solutions might exist, such as a regular powered USB hub or the Audioquest Jitterbug. I didn't try any other solutions. 
 
I can say only that the Schiit Wyrd worked perfectly and solved my problem. 

A short review comparing these to a few other headphones that I own: NAD HP50, Bose QC25, and Sennheiser HD595. Listening via Dragonfly Red and Aune X1s. 
 
These are the only headphones I own that will survive a nuclear apocalypse. They're built so well and feel so good in the hand. It's all metal and leather. 
 
And then Philips goes and blows my mind with the sound these make. They best all these other headphones that I've mentioned in most ways. See my review of the NAD HP50s for a more detailed comparison. 
 
Look, if you want perfect neutrality and a headphone that precisely recreates what the artist recorded, the NAD HP50s or perhaps the Sennheiser HD600s might be closer to the mark. But if you want headphones that just make you groove and bop your head and enjoy your day, buy the X2s. They deserve a place in any collection, especially given the price. 
 
These are fun headphones that make even bad music sound good. Seriously, I'm thinking of listening to Spice Girls and One Direction just for the sake of proving this point. There's something to the sound that is hard to define--I could talk about treble and bass and detail and soundstage, and all of those things are good, but they're all pretty good on the HP50s, too, and I don't enjoy my music near so much wearing them as these. 
 
That's why you buy them. It's not about an audiophile's breakdown of why these are good or bad. It's simply that you stop worrying about what headphones you're wearing or what DAC you're listening to, and you just enjoy your music and get on with your day. 

A short review:
 
This sounds a *touch* better than a Dragonfly Red, significantly better than a Galaxy S6, and *much* better than my Dell XPS 13 (2013 model). 
 
You also get USB, optical, and coax inputs, a quality aluminum build, and a nice, big, smooth volume knob.
 
The biggest downside is that the output impedance is 10 ohms. Given that to minimise changes to the frequency response from your headphones, you should be using an output impedance less than 1/8 of your headphone's impedance, this means that the Aune X1s is best suited to headphones with an impedance of between 80 ohms and 300 ohms. (Why 300 ohms? Because the Aune X1s really isn't powerful enough to drive 600 ohm headphones properly.) That being said, I couldn't hear any changes in frequency response on my 32-ohm headphones between this and the Dragonfly Red.
 
Despite that criticism, I don't care. It still rates as the best DAC I own.

I've tested these HP50s through both an Audioquest Dragonfly Red and an Aune X1S. Both are more than powerful enough for all of the cans I own, the others being the Sennheiser HD595, Bose QC25, and Philips Fidelio X2. 
 
Tl;dr: The HP50s rate second-best in that collection, but are the pair I choose for work and commuting, because they're closed and lightweight.
 
In comparison to the HD595s, the HP50 is reasonably similar in regards to the quantity and quality of the trebles and mids. However, HP50s have decent bass, whereas the HD595s are lacking in both the quantity and impact of that bass. I've read that many people think that the HP50s have a reasonable soundstage, but I could argue that only in the context of other closed headphones. The open HD595s have a larger soundstage. The HP50s are superior for detail retrieval, but it's not as big a difference as you might think.
 
Compared to the Bose QC25s, the HP50s possess more detail all around and suffer less sound leakage. (Of course, the QC25s isolate you more from the outside world, but the HP50s are reasonable given they don't have any active noise cancelling.) The QC25s have one strength--they handle sibilance better than most headphones. While the bass is a little greater on the QC25s, it doesn't kick as sharply, nor does it possess as much resolution. The QC25s are a little dull in their presentation overall in comparison--ever so slightly muffled and veiled. The QC25s are about US$300 now. What you get for that is about the equivalent of a $150 headphone (for sonic quality) with $150 of fancy noise cancelling added in. One other strength of the QC25s is that they are fantastically comfortable.
 
The Fidelio X2s best the HP50s in every way with regards to sound quality, with their downside being that they're very, very open and leak sound everywhere and don't isolate at all. Basically, if I could use my Fidelio X2s while commuting and in the office at work, I would choose them over the HP50s without a question. The X2s are more detailed, have a wider and deeper soundstage, separate instruments more/ are less congested, and sound more forward. I love the sound from the X2s. They make even bad music sound good, whereas the HP50s don't give me the same level of pleasure from a song. The X2s are slightly darker in presentation overall--the HP50s are certainly the more neutral sound. 
 
The HP50s do have some other characteristics worth discussing. The sound--particularly the treble and the high mids--is just slightly veiled, but it's hard to describe this. I've heard it described that it's like you're listening through some cotton balls. But that doesn't quite capture it. The funny thing about this veiled sound is that it's different to the muffled veil of the QC25s or other cheaper-sounding headphones. It does give you the *slightest* sense of what NAD is marketing as RoomFeel technology, which is to say that these headphones give you a *little* hint of sounding more like listening to speakers than some headphones. But what others hear as 'RoomFeel', I initially heard as something like a recession of instruments in the soundstage. I notice it most with vocals. Again, I need to emphasise that this veil is not killing details in the sound, so it's not muffled in the traditional sense.
 
You could argue this as a strength or weakness depending on your preferences. I know that Tyll from Inner Fidelity loved these headphones, and that Zeos from Z Reviews was bothered by that aspect of the sound. Regardless, the effect of this is that the HP50s just present in a very fatigue-free way compared to most headphones. For work, where I want my concentration to be elsewhere and don't want background music fatiguing me, they are a perfect match. However, as mentioned, I just don't get quite the same listening pleasure or excitement from my music that the X2s give. However, the HP50s still rate as the best closed-back headphones I've owned, and you can wear them for hours without listening fatigue.
 
If I had to summarise, it's that the funny thing about the HP50s is that they don't do anything wrong, but they don't do anything that excites you either. Okay, they're more exciting than the QC25s and I can't be bothered with the HD595s any more because of the lack of bass. But that impression of not being exciting would seem to match Marco Arment's thoughts on this headphone as well as Zeo's group review of closed headphones that includes the HP50s.  
 
In terms of comfort and build, the HP50s are reasonably comfortable. The cups fit my average-sized ears, but might not fit larger ears. The centre of the headband can push down on the top of my skull in an uncomfortable way unless I take care to position them right when putting them on. The build is very, very plasticky. That doesn't mean they're not durable or poorly built, but they don't feel expensive in the hand, nor do they instil confidence the way that the metal construction of the X2s does. And while the HP50s look okay from the side, the squarish headband is odd. 
 
They also come with a decent soft case, two cables, a 1/4" jack adaptor, a plane adaptor, and a nice little container to fit these odds and ends.
 
Quality Assurance could be an issue. Mine came with one cup that doesn't rotate in as it should, and the cable with the mic has just awful volume buttons that don't seem to click properly and don't work with Android phones. (Okay, NAD admits upfront that the cable is only Apple-compatible, but it's an oversight nevertheless.) I've also read other accounts of issues with the cables. However, as the cables are standard 3.5mm and replaceable, this isn't a deal-breaker. 
 
Despite these issues, I don't know of any other portable, closed headphones for USD$220 that could compete with the HP50s, particularly with regards to sound quality. (Marco Arment thinks the second-gen B&O H6s are better, but those are significantly more money.) And if I'd never owned the X2s, I would have been perfectly happy with the HP50s. They are good headphones that do nothing wrong, and I think that if you want a closed set of headphones for work and commuting, it really is hard to do better for this price. 
 
Based on sound quality when keeping the price in mind, then they score a 4.5 / 5 from me. But the issues with comfort and quality and design dip the final score from me to 4 / 5. And if NAD ever offers a HP60 or HP70 with a less-plasticky construction, I would certainly be interested and curious.