If I could find my moab sample pack I'd give it a try.
Does it the spread of it match any contours or color changes?
No, it appears to be evenly distributed across the print, regardless of underlying color or tone. My suspicion is that it is somehow being caused by the thickness of the 300gsm paper - perhaps it's running across something in the paper path that thinner papers don't come in contact with (or at least not to the same extent). I thought I read in my B9180 materials that it was ok to use this weight of paper through the specialy media tray, but I'd probably better doublecheck that...