Originally Posted by Gilley

Your friend's assessment of polyfill is backwards. You put polyfill in a box to make it appear larger to the sub by slowing down the air inside the box.

A box that is too large can not only sound bad, but can also be detrimental to your subs.

Originally Posted by Paul350Z

Wholly out of control engineering! Stop before someone puts their ear out.

A good primer to start learning speaker box design is Sound System Engineering by Davis and Davis. Sams published the first edition, Focal Press the second. Older books but good dope none the less.

A sealed subwoofer needs an air freaking tight seal. All the way. Air tight and tighter. The woofer can be driven harder/louder as the air acts as a spring to limit the travel in both directions - the volume of the box has to be correct - the spring has to be the right size to prevent driving the speaker element into the basket or over extending it and stressing the surround.

A free-air woofer is loose and sloppy compared to a sealed box or even a properly designed ported enclosure. It is designed without the external limiting (controlling) influence of a sealed box volume. Better than nothing - and better than a normal sealed box subwoofer in a free-air position to be sure.