Seems so:

http://www.fostexinternational.com/docs/products/TH-600.shtml

"Piezo-sheet inside the housing for dispersion of resonance as well as high damping treatment"

There might be a port inside the cups that may be a little too large, but that's pure speculation. I know there is one inside my HD202s and the Alpha Dogs. In the HD202s it is very tinny and fairly sensitive. Carefully covering it reduces the bass. Totally covering it results in no bass. These are some results of doing just that to my HD202:

I did not noticed much treble peak on this. And I'm very sensible to it. It had fun sound, but not unbearably so. I liked it alot.
About inner damping - I dissambled them, and inside they looks very much like D2000 if I remember correctly. There is also two ports on each cup.

My experience with the TH600 doesn't really match these measurements or the subjective impressions (of this set I'm assuming?). I heard the TH600 as pretty much similar to the TH900 in treble response with a touch less bass presence---or rather the bass seemed slightly more reigned in---and the midrange was slightly less sucked out subjectively as a result. The TH600 sounded less lively and less dynamically apt to me: if the TH900 was a fizzy drink, then the TH600 was the same drink but after being left out for an hour. The TH600 was still preferable to the D7000 for me in just about every respect however.

Didn't MuppetCase say the TH900 / TH600 were the best dynamic driver HP's at one point?

Not easy for me to see in this data, I fail at school.

I said the TH900 (not the TH600) was one of the best dynamic headphones currently in production, and certainly one of the best choices for those who want a clean U shaped signature. I think one needs to look at the distortion measurements for the TH900 in particular to get a better sense of why it's such a good headphone, as the bass is just incredibly clean for a dynamic, and it's not as subjectively bassy as the FR alone would suggest. I thought the TH600 was a good headphone as well after living with it for a few months, though it lacks the TH900's refinement.

I went to a local headphone shop and compared my friend's TH600 with my TH900 out of the Soloist and Bifrost Uber. To me, the 600 sounded way too spiky and aggressive, not in the sense of FR, but the presentation. Listening to vocals, I would cringe at how sharp and intolerable it sounded compared to my TH900 which sounded almost the same FR-wise but less edgy and aggressive. The TH600 hurt my ears but the TH900 clearly didn't. It reminds me of the Gungnir, which I cannot tolerate due to the same aggressiveness which makes me cringe all the time.

I should note that my TH900 is recabled with OCC silver litz. I can't say this makes any sort of difference, but I do find silver plated copper, what the stock cables are, do sound relatively edgy in general. Take it for what it's worth.

The TH900's treble is irritating for me as well after a while, but I find the bass kind of offsets it, like providing a counterbalance on a seesaw.