The receiver shouldn't mind being on the sub, and it should be least likely to be bothered by vibration. I would tend to agree that it's worthwhile to be cautious stacking anything on top of the DVR - my experience with cable boxes is that their manufacturers seem to secretly wish to be making electric space heaters, since ever digital cable box I've ever had has dumped a surprising amount of heat out the vents. The OPPO could go under the DVR and should not be a problem, assuming the Motorola isn't deeper than the OPPO (something that is probably not the case).