Source is a Mac Mini playing a variety of resolutions from 16/44 through 24/192. My musical tastes are very broad and I am open to all genres as long as it is listenable from a production/mastering standpoint (I have ZERO time for "brickwall" mastering techniques so I tend to favour older music!).
It's difficult for me to set the Valhalla in context for you because it is my first proper headphone amp, so I don't know how its sonic signature compares to its competitors. However compared to what I was previously driving my HD600's with (the built-in phones amp of my vintage 1974 Yamaha receiver), the difference really is like night vs day.
The Yamaha is fat, mellow and warm, with a full-bodied/bloated upper bass and comfortingly full mids, with a high end that shimmers rather than cuts. The sound isn't particularly clean, transparent or detailed, and there is a relatively high noise floor of background hiss, but the sound is very musically involving and easy to listen to, especially when kicking back on the sofa late at night and cranking up the volume.
The Valhalla, on the other hand, is very clean, lean, detailed and precise, with a slightly 'brassy' bite in the mid HFs (possibly around the 8k-10k region?). This makes listening at higher volumes more of an "on the edge of your seat" experience and not as relaxing as my old Yamaha receiver. Mids on the Valhalla are much cleaner and thinner with vocals having less weight and warmth but better clarity, and the low end is markedly thinner/weaker but faster/tighter and much more transparent/defined). Also, the noise floor is amazingly low on the Valhalla (my ears detect no his or hum), so it is possible to listen at low volumes and still hear an incredible amount of detail and clarity.
Hope that helps! :)