Getting Sub Deeper?

Polyfill would help a litle bit in producing lower frequencies, but not enough to notice well in your case( the box is right size).

The reason it sounds muffled might be because the box is sealed. A sealed bow produces a punchy bass more suited for jazz and classical.

Porting the box properly will make the more boomy sounds more suited for rap. The "muffled sound is to taste. It all depends on what kind of music you like.

Hope this helps.

As has already been said, using a larger sealed enclosure will increase the low end response of the sub.  Adding fill if there isn't any in there already will help as well.  You could also add mass to the center of the cone which will reduce the Fs.  However you will also reduce the efficiency of the sub. 

I'm a little confused though.  You say that the sub sounds "muffled", which is generally a sign of too much low end (boomy sounding).  The problem might be that you are lacking midbass, or are used to a peakier upper bass that this current design doesn't provide. 

Steven Kephart

Adire Audio

How would you reverse the phase anyhow?