The TH600 is on my head this morning, so here are some very preliminary thoughts. Subject to change 'n' all that.
First I should say these aren't a "D7000 mk2," so those looking for the D7000 sound should try to find a pair of D7000s instead. The bass is definitely reigned-in and controlled more than the D7000, though keep in mind I'm basing this off my experience with the earlier production models of the D7000 and not the later tweaked versions. The difference may be less substantial when compared against the later D7000s. Now compared to the TH900, this is where it gets interesting for me. Could just be a case of wish fulfillment, but the bass doesn't seem as voluminous as on the TH900 either. Unfortunately the bass also seems a little less textured and dynamic compared to the TH900 as well however.
Mids are very similar to the TH900---ie. less recessed compared to the D7000, but still with a slight hint of recession that becomes more evident on male vocals---though again, they fall short of the TH900 in that they just don't sound as open and effortless. AnakChan was correct in his comparison to the TH900's presentation: the TH600 doesn't do imaging and spatial cues quite as well as its older sister. However it's still ahead of the Signature Pros and LCD-2s to my ears. The upper registers are handled well, with a satisfying treble that doesn't become too strident or have some of the glare evident on the D7000, but by the same token is once again falls short of the TH900 in that it falls a wee bit flat, lacking that sparkly effervescent quality the TH900 can have on certain recordings.
Notice the pattern? I'm basically saying these are a step up from the D7000 but a step down from the TH900 in my humble opinion. That's sort of what a lot of folks expected, I think. What I didn't expect however was just how close these come to the TH900 in most areas. From the above impressions, it probably sounded like I was ragging on them a bit, but in actuality the TH600 sounds very good. I'm exaggerating the differences somewhat to highlight them, though other folks may get different milage of course. The biggest difference for me is in the whole gestalt: the TH600 just doesn't sound as "alive" and vibrant as the TH900. It's not a huge difference, but it's there, and it's important I think.
I could perhaps summarize it best with the following metaphor: the TH600 is a softdrink that has been sitting on the table for an hour or two, whereas the TH900 is a softdrink straight from the refrigerator.
Having said that, I think I see where AnakChan is coming from when he says calling the TH600 a "mini TH900" is doing it a bit of a disservice. It's a bit too early for me to be truly comfortable saying this, but I'm hearing some differences between the two in the lower FR. The TH600 is by no means a bass light headphone, but the mid-bass seems slightly less emphasized relative to the midrange. Also as the bass transitions into the mids on the TH900 there's something of a bit of "suck out," and on the TH600 I'm not detecting that so far. If I had to guess, I'd say Fostex might have dialed the bass back just a wee bit because it's simply not as refined as the TH900, so having more of it might end up compromising the overall clarity of the sound which is what Fostex seems to be prioritizing with these transducers. That's obviously just speculation on my part though.
So where does that leave the TH600? The lineage of Fostex's OEM work for Denon is evident as with the TH900, certainly. However I still maintain that those looking for a D7000 replacement specifically would be better served tracking down a used pair (and paying less). The TH600 is still a fun headphone in my opinion, but I can see preferring the more extreme-sounding D7000 for applications like movie watching. The TH600 will appeal to those looking for a headphone that, like the TH900, straddles the line between a more fun approach and a more accurate, reference style of listening. It's a definite U-shape, but it possesses a level of subtlety and refinement that keeps you from feeling like you're missing out on details. You can get lost in the music just as easily as you can sit there analyzing all the nuances.
Finally, a few words on comfort and build quality. These seem to clamp more than the TH900 right out of the box, but overall they pretty much retain that legendary Denon / Fostex comfort. The earpads are thinner and more uniform in diameter compared to the D7000, and they appear to be made from the same synthetic egg protein as those on the TH900 (though they don't smell the same for some reason?). The weird lopsided cup issue that is present on my TH900 is absent on these. The cups themselves are very sleek looking and well made, though their texture is a bit unpleasant to the touch for me personally. I'm definitely partial to the TH900's super deluxe finish.
