I agree with that... Sony CD3K and Senn 600's really aren't in the same league. Sony CD3K is really the next level above the Senn 600's.
True, they are open versus closed designs. However, from my experiences, it seems like although the Senns create a larger sound stage with its open design, the stage only extends from the left to the right.... and you do get the sense you are somehow separated, far away from the sound. It's a very laid back sound experience.
Sony CD3K is much more intimate, however, its soundstage not only extends from left to right, but front and back as well. It has a much more rounded sound extension. With a much more response bass and resolution. It is definitely not an "active" bass as say.. that I've noticed in Grados. It is a very, very neutral reproduction of what the sound is supposed to be like. As opposed to Grado's overly exciting nature, and Senns extremely laid back sound.
Perhaps when Senns 650 comes out, it will be a more adequate comparison between these two phones. However, there's no question which one of these phones is superior at the moment. If price is not an issue (they are in quite different price range as well), there's no reason not to go with CD3K.
Not to mention, the low impedence of the CD3K makes it a much more forgiving headphone than the Senns 600. Senns 600 takes a lot of power to drive to its full potential... Even then, a comparable set-up with a CD3K will sound better.