I'm building another HT subwoofer, this time for my roommate. The box is 4.8 cu ft tuned to 21Hz I believe. (I would have to check that again, I don't have my paper right beside me. I already have the braces planned out for this. The woofer is going to be an SI MAG d2 15" powered by an O Audio 500w plate amp. My question pertains to polyfill though. Do any of you guys recommend polyfill? Also, since this just makes the box ACT larger, does it have any effect on power handling?

Nope.

Nope to adding it or the effects of it?

I don't believe it has any effect on the mechanical power handling of the driver.

*edit* I didn't see the first question about if we recommended it... I don't have a problem with it, stuff to make it work for you is my policy.

that box won't require polyfill. I had that sub in 3.8 cubes in my car and it sounded outstanding. You can try it, but you wouldn't need it to make the box seem larger is my point. I don't think it has any affect on power handling since it's the airspace inside the box that acts as the "spring" to control the cone. The polyfill just acts on the sound waves after they've already been produced by the sub.