I owned a 4 channel B22 and had the Mjolnir in house side by side at one point in time. IMO the B22 beat the Mjolnir out with the HD800s for my preference of music. To my ears the Mjolnir / HD800 pair was to bright. The B22 / HD800 paring was a tad bit rounded. Bass was better on the B22. It's all about how you like to enjoy your music.
A 4 channel B22 is the better all around amp paring with a wider verity of headphones. The Mjolnir is the better amp cost per performance wise. You can't beat it for it's price if money "was" an object. If money "was not" an object I'll take the 4 channel dual sigma B22.