polyfill makes the box "bigger" as far as the speaker goes. if you are using a stock woofer in there dont stuff it. if you are using the Tang Band speaker as mentioned in one of the other posts, stuff it loosely with the poly fill, put a little in the port tube and cut off 3/4 of an inch to tune the box perfectly to the tang band 5.25" subwoofer driver.
I just finished tuning up one of these to match that box. using the port tube with a bit of the poly fill in the tube helps to reduce velocity in the tube and makes it possible for you to tune it to the enclosure and get 35% more bass from it and make it sound better.
UPDATE : I updated fierosound with this info.... Increase the trimming of the tube to 2 inches instead of the 3/4 I reccomended above. It get's rid of some of the boomyness I found. I also added more poly fill to the box as well. I had a problem with a "boomy sound" when anything with heavy bass was played. Taking off the 2 inches made the bass response flatter across the frequencies.
Dont have to take it out, take off the pass side dash speaker and you can release the tube (if you did not glue it on) to trim and replace. That's what I did to tune it further. It seems that the Tang Band speaker will get looser as it get's broken in. this raises the Q and makes it need a shorter port.
[This message has been edited by timgray (edited 05-27-2008).]