I organized a meet the other day and had a chance to compare the HA-200 (monoblock config) vs the Emotiva Mini-X. Listening was done primarily with the HD800 and HE-6, fed from a Yulong DA8. I think it is fair to suggest that the source and headphones used were of acceptably high calibre for such a comparison.
Well, I can safely say that the HA-200 loses out in terms of sheer volume... but for everything else, the Nuforce monoblocks smacked the Emotiva around like an *insert appropriate euphemism here*. Simply no contest at all. Yeah ok it's $220 vs $700, so cost is 3.2x higher*. Would I say it's 3.2x better? You know what... absolutely it is. Even if you can't afford two units for mono, even in single ended use it sounds better. (*coincidence: the gain of the HA-200 is 3.2x). The only hiccup is that if you're trying to push something like the HE-6, you'll need a dac with high output to get sufficient headroom (I suggest 4V).
As much as I like the HE-6, I do think the HD800 is a better pairing (if you twisted my arm to pick one). Actually, in my experience so far I find Nuforce gear tends to mesh really well with the HD800 and gives it a really nice impact in the bass.
I am also finding now that the HA-200 is letting me pick out differences in dacs much more easily.