I have the HD 800 S's and I really liked pairing them with a Modi Multibit and the Valhalla 2. It sounds really, really good, especially for acoustic and classic rock. Honestly pretty much every genre sounds good though since it's such an even sound profile, it doesn't really highlight lows, mids, or highs which I quite like.
I really like Schiit audio, I find most of their stuff to be at an excellent price for the quality. I was recommended against getting the bifrost multibit due to the performance of the modi being pretty good, but from actual users I've read the bifrost has a slight edge in performance and can be upgraded later (however the modi looks much better under my monitor than putting my Valhalla 2 on top of the bifrost, think about your layout too).
Finally Valhalla 2 is advertised as working with IEMs and pretty much all headphones, however I find it best for headphones. Both shure SE846 and Etymotic had a hum, I'm guessing from the tubes but it may be the 3.5 to 6mm adapter since it was the first time I used it. You can really only hear it when volume is super low and I've never heard it on my 800 S's or 6XX's. Also I recommend people read Schiit audios documentation, their website is pretty dang funny