That is one of the best posts I have read yet on Head-Fi.
I guess I shoudn't have had any doubts when it came to Bottlehead gear: the Crack is well-named as an entry-level marvel and at higher levels a true gem with a limited repertoire of headphones.
The SEX just goes to the next level of (ugh) "refinement" in that it really does bring added dynamics and feeling across the board. That's a big blanket statement given just how far you can take a Crack and the many possible sounds you can get from it.
Along the journey I'm valuing more and more that feeling in the real world of sitting in a room with performing artists (and especially musician friends) playing only to me - I hope everyone gets that experience at least once) and the magical moments that can bring. Re-capturing that is the holy grail of music. Not the mega-performance by a rock band in a re-purposed stadium or even a symphonic display from the 33rd row.
Our language isn't great for audio description, and words like refinement (yes I've used it for the S,E,X) are the best we have. Your description helps bring some depth to that few letters. Awesome, and just tells me that the Mainline is where I'm looking to go!
One last thing, out of place, I haven't stopped listening to this partly due to it's "refinement" on the S.E.X and HD-800, and partly because it just screams for it's acoustics and raw musical energy. Turn it up until the hiss is present and really listen to the emotion and "rawness" of the in-the-room experience. The more I get from that the happier a "sono-phile" I am....