On the subject of V6s and CD900STs, I can't imagine them using the same drivers, because they sound completely different. While I can see them both being used in a professional setting, I can't imaging them being used for the same thing.
To me, the CD900STs don't really seem neutral enough for mixing or anything like that. Bass is much too light and lacks extension. Treble also lacks extension, and it's also not really all that sparkly. Just a little dry up top. I could see them being used to track vocals, though, since their mid range is so incredibly detailed, and just true to the singer's live voice.
The V6's by comparison are a bit bassy, and a bit treble emphasized. Despite those two things, I think they are still more neutral than the CD900STs, and could see them used for mixing, as long as you're aware of those (small) issues.
The original 7509s are yet another flavor from Sony in their professional line, but those are discontinued. Those were a neat headphone though. They were much more bassy than either the V6s or the CD900STs, but they also had the nicest treble of the three. Superb extension, with a slight bump in the upper treble. It took a little getting used to, but it gave them a really interesting flavor I've never really heard in a headphone. Ultrasones are similar, but they usually had a slightly lower-range treble emphasis. They also didn't have nearly as nice of mids. The 7509s had an incredibly lush mid range, that never got lost in the bass, and never sounded withdrawn.
I'd really love the chance to hear a pair of Z1000s or 7520s. Sony seems to make a lot of great closed headphones, so I'm always down for hearing more of them.