I am suddenly less excited about the arrival of my Cube tomorrow...

Plan to spend some time with it. If yours works out of the box (i.e. no loose screws, good PSU, full speed) then it sounds like you might be one of the lucky few.
I have 2 cubes and can't get 1/2 (15 GHs) of the advertised 30 GHs (for one unit) consistently out of BOTH of them combined. Yet I have all Os in the web GUI.
I've run into bandwidth issues running multiple cubes on the same proxy or router. Try running a second proxy on another machine for the second cube.