AdamZuf,
I will chime in to somewhat agree with your assessment of the HD600's slight edginess in the treble regions. 5-10k is actually much more of an audible region than the 10k and above, where the HD600 is flatter. I believe that the 5-10k dip creates a bit of edginess to the sound, particularly noticeable to me when using the HD600 with the Cardas cable upgrade. In general, I like what the Cardas upgrade cable does for the HD600, but this is one quibble that I have with it, in that it seems to exacerbate this problem.
I have heard the Sony CD3000s only briefly and have not yet had the chance to hear the Beyer DT880s (although I have great respect for Beyer phones, having owned a 990 for some years), but of those listed, I would say the HD600s OVERALL are the best of the bunch I've heard.
Having said this, I can't imagine either the CD3000 or DT880 being better (or as good as) than my Stax Omega II phones.
I don't particularly subscribe to the idea that more than one phone is necessary for classical due to the different styles, instrumentation, etc. In fact, I believe that finding the right phones for classical is at once the easiest and the most difficult. It is the easiest in that classical music lovers are looking generally for a phone that reproduces the sounds of ACOUSTIC (that is, non amplified at the instrument's source) instruments faithfully, so an ACCURATE phone SHOULD be all that is needed. With pop music, most of the instruments are amplified and therefore it is VERY difficult to determine through listening what sounds most "accurate". On the other hand, finding the right classical phone is most difficult in that classical music is generally more dynamic and contains many more nuances than pop music. So the phone must be able to reproduce these things.
Although there are still a number of amps/phones I have not heard (although I have heard quite a few), FOR ME the Omega II phones when paired with a proper amp come the closest BY FAR to reproducing classical music faithfully. Every other combination I've tried to-date has sounded colored, artificial, and often too bright, especially in the reproduction of string sounds. It's been amazing to me that so many of the recordings I once thought were just too bright (newer ones even) now sound much more natural with the Stax.