It's an excellent stack for the HD800, for sure. The Gustard gear is good for feeding TOTL headphones at a reasonable price, as many have said. You get the quality of equipment that costs thousands of dollars for much less outlay
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You can also kind of extrapolate how good the H10 is with headphones by looking at impressions of the Violectric V200. People on page 17 of this thread talk positively about the V200 and HD800 pairing:
http://www.head-fi.org/t/480038/violectric-hpa-v200-why-not-sufficiently-appreciated/240
My own impressions are very good for the HD800 + Gustard gear. A definite, significant (if you're a good active listener, which you should be as someone who is into audiophile equipment) step up from the Matrix M-Stage amp and DAC, which was once considered the best amp you could get for the HD800 < $1000.
Having said that, the HD800's are my least favourite headphone, LOL. Their presentation of music is, to my ears, un-musical and unengaging for long-term listening. They present sound to the listener in a wide, airy, separated, diffuse and detailed way. Bass is VERY tight, detailed and controlled, but the sound lacks a lot of the warmth that, imo, SHOULD be in music. The sound tilts towards the brighter side of neutral. The H10 and X12 bring some extra musicality to the HD800 sound, while still keeping its strengths. They do not completely stop the sound from being analytical as this is inherent to the HD800's imo.
I think of the HD800's as a precision tool. If you want to analyse music (which I often do), these are excellent. They will lay out details across a large, expansive, airy soundstage with excellent precision and imaging. I do not believe that these headphones portray the music in a way that the artist/recording engineer specifically intended, though - I think that the LCD-X's are much better for this. The HD800 have their own distinct signature sound which is evident in every song.
The thing is, the LCD-2F's, while presenting music in a much more intimate, engaging way, still manage to retrieve a LOT of details and audio cues that make them truly high end headphones.
Then there are the LCD-X's, which are a step up again, managing to combine the sound of the LCD-2's and the HD800's and blending them even further together.
In terms of the technical proficiency of my headphones from the Gustard gear, I would say it goes like this:
LCD-X = HD800 > LCD-2F
In terms of enjoyment, they are all good depending on my mood, but overall it'll go a little something like this:
LCD-X > LCD-2F > HD800
I keep the HD800's because they offer a sound that is so different to my other headphones. Pretty much polar opposite to the Audeze sound. Good for studying music from a different perspective. I would not want them as my only pair of headphones.