Did you ever solve the hum-problem by grounding the ur22 @paulskiogorki?
I seem to be having the same issue here... Just got my ur22 out of the box a couple of hours ago. Everything works fine except that when I turn on 48v phantom power to juice up my Studio Projects B3 condenser mike that low hum appears.
I haven't had the time to try different cables, microphones or anything yet but I did try to ground my ur22 the way that is described earlier in this thread, but it did not work for me...
My Specs: iMac 27" i5 mid 2010, the rest is mentioned in the text above.
Specs: Cubase Artist 7, iMac 27" i5 (mid 2010),
Steinberg ur22, Studio Projects B3
Live long and prosper!