Resurrecting this thread...
Here's my previous "Recliner Rig" which used this chain for the HD800:
FiiO X5 > Metrum Acoustics Octave MkII >
NuForce HA-200 > HD800
(The OPPO HA-1 is not used with the HD800.)
Here's my current "Recliner Rig" (for the past several months) which uses this chain for the HD800:
FiiO X5 or iPad 3 with Pure i-20 > Metrum Acoustics Octave MkII >
Metrum Acoustics Aurix > HD800
(Again, the OPPO HA-1 is not used with the HD800.)
Previously, I had concluded... and posted in several places... that the zero-feedback Metrum Acoustics Aurix had not made significant improvements over the low-feedback, single-ended, Class A NuForce HA-200, for use with the HD800 - that it was somewhat of a toss-up between a preference for front-row seating and a less forgiving, higher resolving sound with better-articulated low-energy transients (Aurix) vs. a larger, but more distant sound stage, a more forgiving resolution and a wee bit more warmth and less "air" (HA-200).
Now, after several months of listening to the Aurix > HD800 almost exclusively, and having established a preference for its 0 dB gain setting (gainless impedance match + attenuation of the DAC, a minimalist's dream come true), I'm discovering that my ears are far more jaded than they were when I last compared the NuForce HA-200 to the Metrum Aurix. No contest!
The gap is far greater than previously communicated, for the same distinctions. Note that I had the allowed the HA-200 to warm up for about 18 hours before testing yesterday morning and I found no difference this morning, after 42 hours of warm-up.
The distinctions I had previously noted, now seem far more exaggerated to my ears, causing me to regret having posted that I could recommend the HA-200 as a better-bang--for-the-buck alternative to the Aurix (for those who prefer a less analytical, more forgiving signature.) The HA-200 is a great amp for the HD800 in terms of eliminating the brittle edginess and thus, fatigue, suffered with high-feedback amps like the OPPO HA-1, but it really holds back the presentation of low-energy information that's so enjoyable - essentially smothering the HD800's resolving power.
Yes, the HA-200 is more forgiving, much more so than I had previously realized, but to put things into perspective, in my opinion, the HA-200 is nowhere near as smothering as the Schiit Vali, the Valhalla 2, or
The Battery DAC, any one of which can make a greater percentage of recordings enjoyable with the HD800, by turning it into an HD600 in terms of resolution.
I've wrestled with the possibility that the Aurix has improved over the months that the HA-200 was in its box or that the HA-200 needs to be burned in again, but I find it far more likely that my hearing has simply been pushed up the ladder of adaptive hedonism.
We know from experience that allowing virgin ears to listen to our gear doesn't always win their immediate desire to upgrade from ear bud-equipped iPhones and iTunes MP3 files, but I'm becoming increasingly convinced that a few months of eating cheeseburgers makes going back to creamed beef on toast a lot harder, but an infrequent taste of rib eye or filet mignon, though pleasurable, is no threat to the cheeseburger. Try cutting out the cheeseburgers for a few months and eating nothing but filet mignon (whenever you eat beef).
Feel free to substitute a different analogy - wine, for example. A friend of mine says he can no longer find an enjoyable red wine under $25 a bottle, and that's an exception. Lesser wines were OK until his girlfriend bought him five cases of that particular wine for his birthday. He drank it like there's no tomorrow and now it's at the bottom rung of his tolerance ladder. He says it's even beginning to lose its grip there, which scares him because a $38 wine sits on the next rung.
For me, the Aurix itself is actually improving with age - requiring a very long burn in to really show what it can do (as hinted by Pjotr27 in his post above) or I am getting "schooled" by the Aurix, as I become increasingly aware of what it can do, like no other amp I've had before. Going back to older gear quite often reveals faults in the newer gear, but that just didn't happen with the Aurix.
Mike