I've listened to around 15 hours on the DX1000 so far. I'd say they are more open sounding than the Alpha Dog and about on par with the TH600 in openness. They don't sound as open as flagship open cans of course but much more than lower end closed headphones, open as in more expansive with a bigger soundstage and a bit more air. The Alpha Dog to me has always seemed very closed and lacking in air despite a more elevated upper end. The DX1000 and TH600 have a larger soundstage than the Alpha Dog, and are wider and especially in the DX1000 case can sound deeper. I feel that the imaging/separation of the Alpha Dog is not as good as either as well. Listening to Electronic tracks on the Alpha Dog leaves me feeling unexcited and flat, while the TH600 and DX1000 are both exciting. On Rock and Metal tracks thus far the Alpha Dog has fared better but I preferred the DX1000 again, TH600 not as much as either. Listening to the Skyrim and LoTR soundtracks I prefer the soundstage of the DX1000 and its tonal balance the most. The TH600 mids are receded and I feel that they sound strangely thin for classical. The Alpha Dog again doesn't make me feel the space or depth of being there with the orchestra, it seems to lack the air which is often a planar problem for me. What I really like the Alpha Dog for though is Jazzy tracks like the Cowboy Bebop soundtrack or the Portico Quartets S/T. The sound of the snare and cymbals come totally alive. Still the DX1000 has that thumping basslines, and smooth musical sound that is so addictive. The Alpha Dog sounds great with stuff like Porcupine Tree, Opeth, Dream Theater, Tool as well to me.
Overall I'd say that soundstage wise the TH600 is the widest with the highest amount of seperation, the DX1000 is the deepest while the Alpha lags behind in both regards. The bass of the TH600 is elevated over the midrange quite a bit and it has a slightly receded midrange which makes the bass bleed occasionally. The bass is very powerful and goes all the way to the sub-bass you can often feel it tickle at times and certainly rumble very hard. The DX1000 bass is also pretty powerful, seal is important here and I need to rotate the headband a little forward for it. The bass rumbles and impacts fairly hard as well but is not as powerful as the TH600, but seems to not bleed at all and is better controlled. The Alpha bass is a fair way weaker than both and lacks the slam both possess, it is very controlled and tight though. The DX1000 mids are prominent and very clear/detailed and have a bit of a euphonic tone to them, much more detailed/forward than the TH600. On certain tracks the mids get distant though, I'm not sure why. The Alpha dog mids are very flat and linear but because it has weaker upper/mid bass it sounds thinner and a bit dry in comparison to the the DX1000. The TH600 still is more weighty even with a receeded mid range. The DX1000 treble is rolled off on my set up and very smooth but not that detailed. Never is it fatiguing at all. I prefer a bit more bite in my treble personally, but the headphone is so enjoyable it is easy to overlook. The TH600 treble is elevated a fair bit making the headphone overall a U shaped headphone. The TH600 treble can be a fair bit "glasslike" or sizzly with the wrong recordings. I hate the treble with lots of my Rock, Metal and Jazz tracks - but for electronic it really brings the music alive with a pop. The Alpha dog is also quite elevated up there in the treble and can be harsh with some recordings that are exciting up top.
The DX1000 and TH600 are easy to power, more than loud enough even on a Fiio X3 and on my NFB 28 I need barely any power. The Alpha is a bit harder to power and sounds it best with 1W + power to my ears. The Alpha is a lot heavier than the other two which are light, but I find all 3 to be comfortable.
Of course YMMV and this is just my opinion. I'm more of a Dynamic headphone kind of guy personally, as I find that Planars often lack the air/sense of space that dynamics have. I'm not sure if I'm crazy or its just me but the DX1000 and TH600 seem far far more exciting, open, spacious and musical than the Alpha Dog which while being no doubt ably a great headphone to many doesn't tick all the boxes for me even though is does some.
I find the Alpha to be about a B for me overall. The TH600 a B+ and the DX1000 to be an A as far as closed headphones go not taking account of open ToTL. I have not heard LCD-XC, TH900 or Signature DJ.
Hope this was helpful.
After you have had more time with jvc's I would really like to hear more impressions how jvc's compare to th600 and alpha dogs. If it's not too much trouble of course 
