My first sub was a Sony and the polyfill helped tighten the boomy sound at least by 50%. As inexpensive as this is, I am surprised that Sony doesn't already include polyfil in their sub! I used the sheet however but I'm sure both work just fine. Rick
Check this thread to see the effect of polyfil/spikes on my SA-WM40. I feel that it sounds much louder and better with the spiked feet and polyfil. I used the loose bag of stuffing, but I'm sure it doesn't matter.