Hi ModestMeowth,
First, I would like to encourage you to try it with your HD800, before listening to me or any other people who have moved on.
I also want to confess that I have a bias against using tube gear, because I immediately fall into wondering whether it would sound better if I were to try these tubes or those tubes, instead of just accepting the amp with its stock tubes. I tend to get obsessive with such things, as evidenced by the many hours I've spent rolling op-amps in my iBasso PB2 portable amp.
But getting back to the sound of the Valhalla 2, in the context of my personal tastes, I appreciated it for its neutrality, its resolution, and most significantly, for how it exhibits none of the edginess one normally hears with most solid state amps driving the HD800. But, as I mentioned earlier, I was surprised at how laid back the Valhalla 2 sounds - as if you are seated well back from the stage, instead of front and center. The sound stage is huge with the Valhalla 2 > HD800, greater than I'm accustomed, from years of listening to the LCD-2 on solid state amps, especially. In time, I could have adapted, I'm sure. Along with the distant seating, however, came a lack of dynamics - a lack of punch or slam. In truth, and I say this with almost a forced humility, that laid back, big soundstage, reduced dynamics sound of the Valhalla 2 (relative to my OPPO HA-1, for example) translates to a more natural and realistic sound than I'm used to hearing. In other words, I'm admitting that I'm probably conditioned to enjoy a level of dynamics and closeness that is quite unnatural - but its what I like!
Again, I'll say, in time, I could have adapted, but I'm on a (ridiculous) quest to push my HD800 in the direction of sounding like the LCD-2. I know I'll never get there, but the Valhalla 2 was a move in the wrong direction for what I'm trying to achieve - except that it had none of the harsh treble that the HD800 is so effective at revealing in my solid state amps.
In short, I think the Valhalla 2 is a great amp for the HD800, but I'm trying to feed the HD800 with a signal that will allow it to retain its many strengths, while also acquiring some attributes of the LCD-2 (as defined by my tastes).
Mike