Actually, you'd need to hook it up to both, along with hooking both speakers up to it. Otherwise, you only get the bass from one channel. I would recommend, rather than hooking up the speaker in/outs, that you hook up the sub to the tape out ports on your receiver (assuming you're not using them.) The downside to this is you lose volume control. But the cabling's a lot more straightforward!

I am the Doctor, and THIS... is my SPOON!