I actually have it reversed.
The CD900ST is more neutral. V6, while fairly neutral, definitely has a recessed midrange. The V6's emphasized high and low ends makes it more fun.
Both have a tendency to show music as it is. I like that trait, but only to a certain extent, and while the V6 is pushing on my limits of acceptable, it remains within them, the CD900ST takes it a bit too far.
*Sometimes I think that I'm so used to the V6 that I won't be satisfied with anything other than "like the V6, but better" as an upgrade as a music enjoying headphone. My only complaints really are really sound stage, and maybe a little more forgiveness for all the horrible audio quality music I listen to.