I've brought home a new subwoofer and for various reasons (kids, pets, lack of floorspace) I would really like to place it off the floor somewhere. I could place it on top of the large bookshelf that my front speakers are in. That would put it between the fronts, but within 8 inches of the ceiling. All the placement diagrams I've seen have the sub on the floor. Is there some reason for floor placement, or do you think that I could get good quality sound from higher up? Thanks