I've been using the Mojo with the HD800 for about a month so far, and I can say that it's a great pairing that has allowed me to engage with my music tremendously. I came from an Arcam irDac + Violectric v200 setup before this - and that was no slouch - but it cannot compare to the level of engagement that the Mojo has brought with it.
Some characteristics of the sound (mostly in comparison to my previous setup) -
The soundstage decreased in width, but the more intimate presentation has more depth to it. Instruments and vocals have more precise placement and separation, and depth is more easily perceived. These characteristics come together to form music that is somewhat holographic and almost palpable. This visceral sense of the music is, to me, what defines immersion and engagement with music.
In terms of tonality, the mid-upper bass seems more prominent than sub bass. This is especially prominent given my previous setup. However, in terms of bass quantity, it is only slightly less than my previous setup which is likely to be more bassy than what people are used to from the HD800 anyway. This somewhat lends itself to the midrange, which is more thick and lush, which to my tastes complements the HD800 superbly. In fact, if one is used to a leaner sound, it may take some adjustment to appreciate this lushness. Or if one is strictly into a leaner sound, maybe this pairing will not be ideal.
The highs are something I've taken a longer time to appreciate. Previously, the Arcam + V200 setup had a smooth and laid-back upper end. The highs are well extended but also more smoothed over. They are not laid-back and are in fact quite prominent in the spectrum of things. It does not get sibilant, but I found myself becoming fatigued more quickly than on the previous setup when listening at similar levels. I've become more accustomed to this, however, and I've found that listening at lower volumes has brought me more enjoyment from my music while reducing fatigue. I hope this isn't misconstrued as sibilant highs forcing one to lower the volume. I could just as well listen at my prior listening levels, but to my ears, the mojo sounds even better at lower volumes.
Personally, I find the imaging/depth/engagement of the Mojo most appealing when used with the HD800. Coupled with it's pleasing tonality, I have had no regrets with the current setup. Not to mention that being able to enjoy this setup from any part of the house - away from my desk - is incredibly liberating.
I hope this gives you a better idea of the pairing!
On a side note, I experimented by lining out the Mojo to the Valhalla 2 and felt that it made the sound less appealing. I felt that the imaging was less sharp and focused, the bass became too loose (though improved with non-stock tubes), and the depth/engagement I previously felt could not match the Mojo alone. These comments are not meant to be incendiary, and are just my perspective of how things sound. YMMV of course!