I've got a set of 200W Sony Xplod 6x9 on mine with a goodmans head unit, the speakers kick the bass well up, and there isnt a peep out of the shelf even on low volume. The shelf shouldnt rattle at all with the boot closed, there should be 2 rubber stops on the edge of the shelf to hold it in place with the lid closed, your mate might just need a bit of insulating to hold the shelf in place??