I own the prvke 21L: the camera cube is well designed, the roll top is very useful to increase the bag's capacity, I find it very confortable to carry but I gotta admit it's on the heavy side and I wished the higher compartment allowed for more organization (but I understand it would get in the way of the "Pro Camera Cube")
Edit: oops, I understood you were looking at the prvke 31L, not that you already own it, my bad!