Obviously if a sub is down firing, the floor bounce is part of the design. But wait, why do they sound better upside down? Or why do front firing subs sound better when raised off the floor a foot or so?
The traditional wisdom says put a sub in the corner. Nay Nay, I say. Put them in between your speakers away from a corner, and get them off the floor. You may have to adjust the levels a bit, but they will sound faster and cleaner and probably integrate better. Getting them away from a boundary will reduce that boom, making them more articulate.
Try it.