Pros - Good balance, intimate soundstage, easy to drive
Cons - Slightly on the bright side, not extremely resolving
I bought this headphone last year from a fellow head-fier. Unfortunately one of the channels recently died. I suspect this is a cable issue as it was recabled by the first owner. Eventhough at that time I was already looking for a better closed headphone to replace them, I must say I will regret the CD900ST. This is a very capable headphone for the money.
As a studio tool you know what to expect, and the CD900ST does deliver a very well balanced and smooth sound. Unlike many headphones they don't have forward midbass, upper mids or treble. The only other fullsize headphones I've heard that are so well balanced are high end stats (the CD900ST actually reminds me of the Sennheiser HE60). They have an airy quality that can make them brightish and inconsistent sometimes, but they don't have a peaky treble at all. I know there are many conflicted descriptions about its treble, some people call it bright and analytical, while some others call it dark. I don't think these are accurate statements, again the CD900ST is quite flat and has an airy feeling that do make it sound a bit analytical, but not agressive by any mean.
The only downside is that you could sometimes wish for more bass quantity. Bass extension is good but bass and low mids are a bit on the light side. As a result tonality is not the most realistic I've heard, and I prefer the darker balance of the Stax SR-007 and SR-003. But if you can get used to it the CD900ST can become a very addictive headphone. It has a very intimate soundstage that sounds super clear and engaging. Female vocals can be breathtaking. Bass has good impact and music always flows in a nice airy way.
You should keep in mind that CD900ST is not a giant killer, it's a modest studio headphone, as such it doesn't have extreme resolving capabilities. It's not fast and razor sharp like an electrostat or even like balanced armatures IEMs. But for the money you will hardly find a better portable headphone (it beats the HD25 hands down). Get it recabled and you have a killer portable can. Comfort is quite good for me and weight is ridiculous. I should mention that it's also very easy to drive and and can sound very good out of a cheap Sansa Clip. Warmly recommended.
Addendum: I can have my CD900ST repaired as it was only an issue with the jack plug. I'm glad and this headphone is a keeper at the moment... I've had two occasions to audition the Z1000 lately and I was underwhelmed, I consider the CD900ST to be more balanced and clear, and maybe as detailed. The Z1000 can hardly be considered as a studio reference. I also let my Thunderpants go without regret. These were far from neutral and not especially impressive from a technical standpoint (in other words: way overhyped !!). The only thing I really liked about the TPs are soundstage and presence.