Before messing with a sub, I would suggest taking some rta measurements of the room, to see exactly where your speakers roll off in your room. Then, you can decide how to best go about integrating a sub, and objectively what you want to accomplish with it.
Subwoofer set up
How can I effectively use a subwoofer when my speakers go down into the range of the subwoofer. If the speakers are connected to the output jacks and the subwoofer to the subwoofer jack, both units will be reproducing the same frequencies at times. Even my bookshelves claim to produce lower freqs.