I don't think there is an option to do so, besides the iSub, which you could give a shot given they go for quite low prices on the web such as ebay. I would personally upgrade the overall sound; buy external speakers. Really good ones are as follows: Altec Lansing FX6021, Bose Companion series (choose your pick, fantastic sound from all models), Klipsch ProMedia, Acoustic Energy Aego, and the Logitech Z series. If you were to go with the Logitechs, get anything from the Z-4 and up, anything else is crap. The are are my favoritre, I recommend these the most. They provide really great, extremely natural, accurate sound, while being compact, very well built (I've dropped the sattelites numerous times on from over three feet high onto surfaces of wood, concrete, and carpet, all of which were accidents, except for the carpet
; was demonstrating to my friend how solid they were, though carpet wouldn't do much anyways
he got the point
), and the Bose still perform like new! In addition, Bose has some of the best customer service from any company. One of the few very respectable, well accomplished companys out there, IMO.
I suggest that you go out and audition all the systems you can, preferebly the ones I listed above, and choose which ever one sounds best to your ears
Hope this helps!
Peace