I bought the CD900STs around 2 years ago while I was still living in Japan, so I thought I'd write a bit about them.
The MDR-CD900ST is an OEM product made especially for Sony Music Entertainment (Japan). It comes in a plain, flimsy, unmarked white box, and other than a clear plastic bag, there is no other packaging/insulation material. Because it is a SME product, warranties are handled only by SME, and the box contains no warranty card. Your reciept is your warranty.
I'm not sure about Sony Corporation's studio monitor headphones, but the MDR-CD900ST appears to have a built-in crossfeed. I say appears, because I haven't actually openned it to see the insides. It DOES have an unusually stable sonic image centered inside your head, and during burn-in, the frequency response of the 'phones changed to an abnormal extent (a complete lack of midrange first, then later as burn-in progressed, ridiculous amounts of bass, or something like that. I forget exactly now). The hard left/right placement of sounds also seems to be reduced with these 'phones.
The headphone is consistantly cited by various Japanese audio magazines as having a sound image similar to what you hear with speakers. It is true that these headphone don't have that in-your-face presentation that's a feature of a lot of headphones, and again there is that stable centered image. On the other hand, I find they lack the low level resolution needed to get a sense of any soundstage information.
Compared to other headphones I've tried like Grados, and the MDR-CD3000s, these headphones seem to require amplification with a lot of control. I've actually had the best luck with the speaker/line output of my DELL dimension PC. With any other amplifier, even an AD823/EL2008 combo, the 'phones lack life. Aggressive amplification seems to be the key to getting these things going. Used with the headphone outputs of my D-777, the phones are emensely dull, with absolutely no sense of pace. Used with the right amplification however, these phones have a very solid, bold, balanced sound.