Just to chime in, I have just recently purchased a set of DT 880 600 ohm version and with the Valhalla the pairing can be sublime. I say can be depending on the sources i.e. cd player and music. I had been using the Valhalla with my HF2s, 32 ohm and was told the synergy wouldn't be as good. Have to say I love the combination, but as you would expect when using the DT 880s with the Valhalla to achieve the same volume level the gain has to be raised.
With the HF2s I rarely exceeded the 1:00 position, however with the Valhalla and older recordings I found the gain at 3:00 and even full with some 80s discs. I will say the most extreme gain needs came when I was listening through my brothers Harmon and Kardon CD player as opposed to my Rega Apollo which I think must have a stronger/cleaner output.
Anyway, I guess my point is that the Valhalla is a fabulous sonic match, but just a little underpowered for the 600 ohm with some material. When the source music is loud enough, while still having dynamics, the Valhalla/DT 880 combo is golden. I also really love the 880 through my Creek Evolution integrated which I assume has a good amount of voltage, I'll have to check on that.
I can't wait to get to know the DT 880s better, they seem so well balanced, perhaps mids a little bit back, but I have been on the Grado sound for years (which I very much like) so I guess it may just be that I am used to more forward mids? Cheers.