This isn't entirely about the DX000 line, but it relates - So the metallic sound I mentioned with my D2Ks, I noticed I hear it in the Fostex TH600s as well, though definitely not as much. If I'm playing loud music for example and then instantly mute the audio via Windows 7 mute, or really just in some tracks without extra effort -- I can hear it. It's like I am hearing the residual compression sound or something after the drivers have expanded to pump out the audio. Exact same metallic tinge as my D2Ks, but it subsides much faster where as the Denons ring for just a few ms longer.
Is this a fault in the DX000/THX00 lines, inherent nature of closed-back cans, or maybe just something my ears are sensitive to? Happens with or without my amp, regardless of audio source. The only other headphones I have are the Sennheiser PX-100s, and I do not hear this metallic sound at all with them.
As it was one of the primary reasons I decided to invest in the TH600s, I'm considering returning them now.