That's a pretty wide range spec for the sub..... Like I said, the polyfill is mainly used as a fix for not having the airspace the sub is designed for(say, a sub needing 4 cubic feet of airspace in a truck box in a reg. cab). So as long as your box meets the airspace dimensions the polyfill is gonna do nothing but dampen the sound. The way it works is it slow down the sound waves coming from the sub, tricking the the sound waves into thinking they are travellign farther, and that the box is bigger. So if the box is already big enough, and you throw some of that stuff in with the sub, you are basically putting it in a bigger box, causing the sound to trvel farther in the box and damping it out more before you actually hear it. This may help your problem with the reflections mentioned earlier but I wouldn't go crazy buying the stuff. It is rather inexspensive so it may be worth a shot, but I think your best bet is gonna be changing the position of the box. Just my 2 cents.....