Bought this to use on the go and couldn't be happier. I've compared it to both the Dragonfly and Hegel Super and I can say that it comfortably trumps both those. The Dragonfly is good but music from the Explorer seems a bit more easy going and natural in comparison. And the Hegel Super is sadly disqualified for having a very high noise floor, on high impedance headphones you can't hear it but on everything else a very audible hiss is present.
 
I've also compared the Explorer to my Anedio D1 stationary DAC, and for a $300 portable device it does exceedingly well. As one could expect there is a performance gap here, and in comparison the Explorer comes across as more "mid-fi" rather than "high-fi" in flavor, but that's not always a bad thing depending on what you're after. It wasn't that long ago that this level of performance cost three times as much and only came with a power cord, for a USB powered portable device it is seriously impressive.
 
I also tried to power my HD650's from the Explorer and was very pleasantly surprised, I've heard several wall powed headphone amps that have done far worse! For such a puny device it has a very impressive drive and bass definition. If I was shopping for a entry level headphone setup a Meridian Explorer and HD650 would definitely be on my short list! It is hard to understand how well this little device actually drives these headphone until you've heard it. I suspect they must have used the HD650 as one of the development headphones, the synergy is that good.
 
Meridian has a long standing reputation in the Hi-Fi industry and the Meridian Explorer has been recieving a lot of praise in the press lately, but I'm happy to report that in my opinion all of it is well deserved! The combination of price, performance and form factor is very attractive and as an audio upgrade for a travel laptop it's almost a no-brainer. In short, this little device rocks and I have no problems recommending it.

I've owned the HD800's for the better part of two years, by now I have hundreds and hundreds of hours of listening time behind me. It is never easy to review a flagship product but the HD800 is harder still, it is not a random thing that I've held off doing this review for so long.
 
The first time I demoed the HD800 I was of course incredibly exited to try Sennheiser latest and greatest headphone, who wouldn't be? I was impressed by the build quality, the "all-day-long-disappear-on-your-head" comfort, the attention to detail, the very German "rightness" of it all. As I listened to some of my favorite demo tracks in the hi-fi store I was amazed by the details, spacial presentation and precision. But at the end of it all I also came away more than a little perplexed, and there was no instant buy that day. Some of my favorite recordings had sounded a bit dry, hollow and sterile. Almost as if they had been stripped of emotion.
 
So at the time I simply concluded that the HD800 was competent, but that it wasn't for me. And no great loss since I carried on with my HD650's, a pair of headphones which I to this day still hold in very high regard.
 
But after a year or so I started thinking about the HD800's again, I guess curiosity is the only way I can describe it. Was I sure there really wasn't anything special with the HD800? This was Sennheiser we were talking about, could Sennheiser really be wrong? So I started reading about the HD800's, and how divided the opinions on them were. This didn't seem to be a "normal" pair of headphones which you could just demo for an hour or so. These headphones required you to actually buy them, install them in your system, set them up the way you want them, let the headphones break in and then let your ears acclimatize themselves.
 
But even armed with this knowledge I still wasn't completely sold on buying them, it's a lot of money to put down on the table when you're not sure if you're going to like something or not. But one day I spotted a mint second hand pair from a professional reseller at a price that was just too good to pass on. So I decided just to go for it -  "heck for that price I can probably sell them for the same again if I don't like them" I said to myself.
 
And away from the pressure of the demo room I slowly began to warm to the HD800's and appreciate what's so special about them. This isn't really a headphone that's built to be a headphone, this is a set of headphones that's supposed to sound like a pair of speakers standing in front of you.
 
I have never before or since had a pair of headphones that image so well, it is simply uncanny and something that has to be experienced. Since they are so transparent they also respond extremely well to tweaking. If you change a cable, change the source or give them a +2 dB bass boost you can really hear the difference. This is both good and bad; it is good because they respond very well to changes down the line. It is bad because they are extremely demanding; if you have unclean power in your home, a low quality DAC, a humming amp or a low quality recording you will hear it.
 
And in this regard the HD800's are extremely unflattering, they will show absolutely no mercy towards bad recordings. On the other hand; if you give them top-notch components and a good recording they will blow almost everything else out of the water. This is why I like to think of the HD800's as a highly strung F1 car. They can be a pair of demanding son-of-a-somethings, but if you get them set up right... boy do they perform.
 
As such I highly recommend the HD800's, I've derived tons of listening pleasure from mine. It is a headphone that I like to think can do things which no other headphone can. But it is also a headphone that's best purchased when you've already owned a couple of other headphones, and you already have a very competent DAC and AMP setup. Even then it might not be a pair of headphones that you want to use all the time, or for all your recordings. But with the right setup, and the right recordings, you can experience "moments of greatness" which few other headphones can match.

In one word: Stunning.

Not for those looking for a colored and romantic tube sound but for everyone else it's hard to see how you could go wrong.
 
The D2 has balanced outputs and a slightly better headphone amp but if that doesn't interested you a second hand D1 is excellent if you can find one.

I've listenend to all the top offerings from Denon, Beyer, Audeze, AKG, Hifiman, Ultrasone & Sennheiser and this is one of my personal favourites. It is a true reference headphone that is very natural sounding, albeit leaning ever so slightly towards a darker tone. The sound is detailed, smooth and richly textured, especially drums and vocals come alive amazingly well. It has no problem spanning all the way from the deepest bass to the highest highs with supreme control and plenty of muscles to spare.
 
For the money I think this is a real audiophile bargain; the HD650 exists on that plane of ultimate performance where musical enjoyment no longer becomes a factor of sound quality, but rather personal taste. It really doesn't get much better than this no matter what you pay. The HD800 has better resolution, speed and spacial presentation - but can be a bit cold, analytical and unforgiving. The LCD-3 is arguably a higher-performing heaphone, with amazing resolution, extension and control, but in my humble opinion it can sound a little dry and laid-back compared to something like a HD650, Grado, Hifiman or D7000 which are more "fun" and energetic and with a more "forward" sound.
 
So in short - the HD650 I think is truly great, not unbeatable in specific genres but an amazing allrounder. It should however be said that it only truly begins to show what it can do with top notch source and amplification - I wouldn't recommend driving this beauty with anything like a phone or Mp3 player due to its rather high impedance and revealing nature.
 
Although "revaling" is also a relative term since they are infinitely more forgiving towards bad recodings than a HD800 or LCD-2/LCD-3. It should also be said that I do think they exhibit a slight veil (this eternal debate) but ONLY at low listening levels - so that those who find themselves always listening at very low volumes might want to also consider something along the lines of a grado reference headphone which are known for their "intimacy".
 
All in all the HD650 is a superb headphone that comfortably matches, or beats, competitors at more than three times its price (not joking). A superb allrounder with a fun and energetic play-style, detailed but not too unforgiving - not to be overlooked! Seriously.
 
Edit Sep 2016: I've increased these back to 5 stars again. After changing DAC and some other system tweaks I managed to elevate my HD650's to a yet another plane of performance. They just scale like crazy and completely unlike any other headphone in this price range with the exception of the HD600. Sorry for the confusion.