Yeah, the problem I have with the TH600 is if I am mildlly disappointed I would just send them back and get the 900 anyway, and if I love them and get a taste of hi-fi I would probably just start lusting for the 900 yet again. Trying to avoid that middle ground haha.
The best thing that could happen would be I got the 600 and absolutely was not impressed...then my lust would disappear and I could go back to being content with mid-grade IEMs
another option for classy audiophile bassheads is the ultrasone signature djs at $1000. they seem to have a intense following in the basshead community.
closed, portable vs the open, non-portable th900 so may be more suitable depending on your needs.