I just received my Valhalla, and am very impressed. It has only a few hours on it, and already sounds about as good as my Single Power PPX3. The PPX3 has been my reference, and is easily better than numerous other amps I have owned, both in terms of sound quality and noise performance.
It will be interesting to see if the Valhalla can overtake the PPX3 (as right now they seem to be neck-and-neck). They have similar sonic signatures, and I gather this is the case because they have similar designs (single-ended, class A, and such). At $349, I cannot imagine a better choice for a 300-ohm headphone. The designers seem to have had the Senn 580/6xx in mind when they designed the Valhalla.