The issue...I had a 5.1 Home Theatre in a Box speaker set that was of good quality (made by a company that specializes in speakers) that had a built in receiver/amplifier. This system worked fine for several years until I got a new 7.1 receiver. All of the other speakers from the system hooked up as you would expect to the new receiver, except for the subwoofer. Seems that today's surround receivers require a powered subwoofer as part of the system. The subwoofer from the HTIB was in perfect condition and sounds fine as is, so the solution became to add a separate amplifier to drive it.
Step 1: Remove back of subwoofer
My sub had the receiver/amp built in to the back of it. I removed these parts since they weren't being used and found the two (+ and -) wires to the speaker.