quick question: which subwoofer?

Haven't heard either. But I would suggest you try the one your dad is willing to give you, then buy the Polk locally if you can and compare the two. If the Polk isn't that much better ... take it back.

At this price range I do not think there would be much difference between any manufacturer of sub-woofers.

I would say get the free one.

That Yammy actually goes slightly lower than the Polks though specs dun tell the whole story but the I/O looks fine for most configs so give it a shot hehe