OK, been a while coming, and I only listened to about 1 1/2 CD's carefully.....
But the Neko D-100 Mk II --> Black Diamond --> Senn 800 is nearly brilliant!
Jack, your description of dark and effortless for the Neko is pretty spot on IMO. Combining it with the BD yields more of that same impact. I DO think there may be a very slight midrange presence/push, but it makes listening to vocals quite nice IMO.
I listened to some cuts from the following CD's:
Livingston Taylor - Ink - excellent recording by the Chesky folks, Liv's voice front and center and super-clear. Plucked guitar strings, percussion, all pretty transparent and spacious, with the vox still a bit forward. But it's a close-mic'd recording on the vox so I expect that. Effortless is a great word to use here, the music just blossoms out of space
Loreena McKennitt - An Ancient Muse - more loveliness in voice, not quite as forward as LT but MAYBE a bit more forward than I'm used to. Some good tight bass, an wonderful varied assortment of instruments (including a nice hurdy-gurdy), and a fairly spacious recording. My Auralic goes even deeper for presence and space, but the Neko at almost 1/3 the price is just great
Shpongle - Nothing Lasts....But Nothing is Lost - great dynamics, high end clean and extended but not too harsh; maybe some tiny roll-off in very highest frequencies (could also be my hearing, last i was measured about 1.5 years ago I heard OK up to only ~17kHz, maybe 4+ years before that i was at almost 20kHz!). Recording doesn't do a ton for acoustic space, but little sounds placed all over the limited headstage were reproduced quite clearly
All in all a great combination I'd say. I was wondering if this would be too much of a good thing, that maybe something (like the grain-free you mention, Jack) might be too much if both units had it. But I think the BD keeps all the strengths of the design, but doesn't exacerbate anything that could make it a bad combo.
Now that I've moved the BD amp to the system with the Neko, I may keep it there and listen more this weekend.....but wanted to get the very good first thoughts out.