If I am not mistaking (and someone correct me if I am) I am pretty sure when down firing the best way is to use 3 "walls" as legs (basicly leaving one opening facing whatever direction works best in your setup) and the sub should be off the floor aprox. a height of double the one way XMAX of the sub. So basicly if you your sub has an XMAX of 20mm (.79" roughly) your legs or "walls" should be roughly 1 1/2"-1 5/8" tall.....