Efficiency is more a factor. If you go very large, power handling will fall off, but if you stay close to suggested that is not an issue. Also, don't really need to worry about standing waves in a sealed box that I know of. I would not build to account for poly-fill. I get the idea you have this impression that if the air space a little larger that the sound is not going to be crisp and clean, that is also not an issue when the Xcon is in a proper enclosure of suggested size.