Thanks again to Justin for the opportunity to demo these cans in my own home with my own equipment and compare them against my own headphones.
What can I say, the TH-600 are undoubtedly a world-class headphone. They are spectacular. They have a huge and yet immersive soundstage, the best I've heard so far (mind you I haven't heard the HD800 or T1 yet). The center image is stretched out naturally in front of you. Instrument separation and positioning are very, very good and much better than my HD600 or DT880, especially in the center image. The sound signature was surprisingly neutral on the TH600. The bass is emphasized but its not overdone and although I can see why some have called it boomy I don't think that's an accurate word for it because it is only ever so slightly boomy. It is very good quality bass that is extremely satisfying and it stretches way down to the very lowest registers. Other than the bass, the rest of the spectrum sounded very neutral, actually, with no particular area taking over. Details and substance were abundant from the low mids through the high treble. I wouldn't say these are bright at all. Some might say they are warm. To me they were quite neutral and it was obvious that nothing was missing. Once again my experience with a headphone's signature vibes quite well with Tyll's measurements. The measurements show no significant peaks or valleys and that was exactly what I noticed and I came to appreciate it very much. End to end they are smoother than both my DT880 and HD600. In fact, after packaging up the TH600s on Saturday to send them out I started playing around with EQ and was actually able to get my DT880s to have a similar signature. The spacious and accurate soundstage isn't there but the DT880s are very pleasing with this signature. The slightly emphasized but not overdone bass and smooth treble and mids of the TH-600s is pretty much my perfect signature, at least for an all-around headphone. If you like good, strong bass I could easily see this being an end-all headphone for a lot of people.
Earlier I mentioned the one weakness of the TH600-- isolation. It just
barely isolates and only high-end noises. There's not much more isolation here than in my semi-open DT880s. This is the only reason I could see someone not liking the TH600 unless you just don't like the signature at all.
I did perceive slightly more grain on the TH600 than on my DT880s, about the same amount as my HD600. I wonder if the TH900 improves on this at all.
I absolutely loved the TH600s and hope to own them one day. If you are considering the TH600 and enjoy a slightly bass-emphasized signature, take the plunge, you absolutely will not regret it. These are very impressive headphones that beat every other dynamic 'phone I've heard in almost every way.
Here are Tyll measurements for the TH600:
http://www.innerfidelity.com/images/FostexTH600.pdf
Here is the TH900's signature compared to the DT880 (the TH600 and TH900 have very similar signatures):
And here's the EQ I've come up with over the last couple days for use with my DT880s. I've come to realize that the emphasized or at least neutral sub-bass of headphones like the TH600 and my DT1350s is pretty much a requirement for getting the right atmosphere out of some of the genres I listen to. The same way well extended highs can give you the right amount of top-end atmosphere, well extended bass can do the same at the low end. With this EQ the DT880s sound more like closed cans in regard to bass but the soundstage hasn't been reduced at all. The bass isn't quite as emphasized as on the TH600 but is compensated to be neutral all the way down to 20hz. Slight tweaks to the treble spike let mids come through better and also impart a slightly more accurate tone to the high mids and low treble. Thanks to the TH600 for serving as an example and providing me with the motivation to find the signature I've always wished my DT880s had to begin with (because I love all their other characteristics):
