actually the best Amplified Sub I have heard is the Infinity Basslink it is fantastic for someone on a budget but wants a nice tight sound. I know there are people here that have have stuff that will blow it away but the Basslink is a well made quality product. I was stunned by the ability of it.
I've got 2 10' Sony Xplods(not the cheap kind at walmart) mated with a 1200 watt amp. The subs can handle 1100 watts peak each which blows all others in the price range away. Stop by your local Best Buy they have pretty good deals.