Loved the LCD-2s balanced off of the S.E.X. Hearing that combo at a meet made me get the S.E.X. Am enjoying the S.E.X. immensely with my HD800s. They do well with the Fostex TH900s, but because they are relatively sensitive, there is a low level hiss when music isn't being played. I thought the HE-6s sounded pretty good with the S.E.X. as well - no lack of bass, treble was well behaved. But I haven't heard them out of some of the speaker amps that people are trying. HE500's were great. I believe that some amps might do better with different headphones, but I haven't found a headphone that didn't sound at least very good with the S.E.X. The Audio Technica W3000ANVs were very good, too, just not for me.
I am making a pair of Fragglepants (Thunderpants clone) and will be testing those hopefully this weekend.
I might be too easy to please, but I think the S.E.X. is an astoundingly versatile amp and easily one of the better values in this hobby. I have been fortunate to hear some very nice equipment and I have yet to feel like I am missing out. (Well, the SR-009 out of a Ray Samuels A10 was pretty amazing)