Here's a quick summary on the HD600. I am going to leave out headphone specifications because you can find them on Sennheiser's website. These are open cans ,not suited for portability, and they do require amping (300 Ohms).
They are very comfortable and my ears don't get too hot after listening for quite some time. The cups are quite deep and I like it that way. I had hours of listening enjoyment with it and so far had no problems with comfort.
Wow, these cans sound completely natural! I know the differences between natural and neutral but the HD600 has both elements. Although Sennheiser has been long associated with a veil, the HD600 does not seem to have any. SQ is completely transparent and I can assure you that the HD600 has no veil at all.
The HD600 is very detailed although not up to DT880's level of detail. For classical music, hands down the HD600 is the king. Violins are wonderfully textured and have a fantastic weight on them. Highs and mids were fantastic and the bass has great impact and that nice punch in them. The HD600 is no slouch, it is aggressive and sound very neutral and balanced across the frequency range.
It did a great job in instrument separation although the size of soundstage could be bigger. Don't get me wrong. Its soundstage is fantastic and has the characteristics of an open can but vocals are more upfront and intimate.
Quick Comparison to the HD650 and HD800
Neutrality: HD600 > HD800 >>>> HD650
The HD600 is the most neutral among the three cans although as mentioned the soundstage should be a tad bigger. HD650 to my surprise is dark sounding, veiled and coloured (in a good way). It has more bass than the other two headphones. The HD800 has slightly too much treble in my opinion.
Soundstage: HD800 >> HD650 > HD600
No challenge here. The HD800 is the clear winner.
SQ: HD800 > HD600 = HD650
The HD800 has a much better presentation and detail retrieval than its younger siblings. On the HD650 & HD600, it's a matter of preference really. If you like listening to pop/rock, it's best to stick with the HD650. And if you like uncoloured headphones or listen to the music audio engineers intend them to be, go for the HD600 and ultimately the HD800.
Amazing cans and they deserve the title of a legend. They are worth every penny and imho is a top tier can. Comparing them to the Beyerdynamic DT880, I cannot say that they're better/worse than them but all I can say is that both cans are one of the best headphones out there and they compliment each other. When listening to classical music I always reach for my HD600. They are simply astounding and sound heavenly in your ears.