Is there any subwoofer that will just work (plug in) to the existing Klipsch RM-15 Bookshelf Speakers that I have? I have the two of them connected to my PC and they sound fantastic, but they could use the addition of a subwoofer. I see a couple of lower priced Klipsch subs that would do the trick if they work. Does anyone have any experience with this? Suggestions are appreciated.