Yeah! That's what I'm going for, Armaegis: jaw-dropping. Haven't heard the Sextett. Can't remember how they were said to have sounded at the moment...but I have indeed been hearing about the synergy between the Crack and the HD600.
Anything that improves on such a fine set of cans to the point of dropping jaws, and it's definitely my cup a' tea.
Whirlwind -- I'm definitely liking what I hear about the MAD EAR and Grados; it'll probably be in my future. I'd just like to test the waters and see if someone has great luck with a set of cans not already in my collection as well.