Hi Ricky:
The dampening material can be used to tune the sub in your wall. I would experiment without filling, then with filling to see which you like the best. Typically fiberfill is added to make the Subwoofer think it is a larger cavity. The SMP500 would be the best for the Subwoofer especially if you want to power two subs. You would need to run them in series so the amp would see an 8-ohm load versus two in parallel since that would drop the two four ohm speakers down to a 2-ohm load which it would not like. The biggest bang for the buck would be purchasing 4 of the Subs and wiring in Series-parallel, so the amp sees a 4-ohm load. It drives up the sensitivity (making the subs much more efficient), and now you have the cone material of eight 8" subs. Then you are getting the most out of the SMP500.
TL/5320