A local buddy brought over his Schiit Mjolnir a couple of weeks ago and we listened to it and my WA22 through my Audiophilleo2/PurePower and Yulong D18 DAC. We listened to T1, HD800, LCD2.1, and HD 650.
My honest impression is that the Mjolnir was very impressive and for the price is a much better bargain than the WA22. I preferred my WA22 by a tad, and my buddy preferred the Mjolnir. The Mjolnir was so good for so much less than if I had to start over and buy something I might go that route instead. However, I love my WA22 so much I'm not quite willing to sell it and buy the Mjolnir to save a lot of money. If you already have the Mjolnir, it might be tough to spend all of the extra money to upgrade since I doubt the performance improvement will feel as significant.
Then again, my Yulong D18 DAC is very well known and popular for providing a smooth and warm sound that is still very detailed. I think that my DAC really helps the Mjolnir. If your using a DAC that is more analytical, the Mjolnir might sound a little cold and uninvolving, which might make the WA22 feel like a bigger step up. It's tough since there are so many variables here, but I hope the general impressions help.