Generally speaking you usually run your right and left channel speaker wire into the sub (say two 6' pairs) and then run them out to the Right and Left channels speakers. This type of sub will only give you so much- don't expect too much in the way of actually deep bass. I have not heard of after market "in between" amps for this application. That's why you'd go with a powered subwoofer. Try the above though and It should work fine.

You will have alot of options for a subwoofer amp if your receiver has the correct outputs and there are some work arounds if it doesn't. What kind of outputs does the receiver have, sub out RCA jacks or just sub speaker wire terminals?