If you search eBay you'll find plenty of lone subs for this system. Just make sure you don't accidentally get one made for the 4.1 or 5.1 systems, those systems were both flawed.
"Oh, your birthday is coming up? I'll make sure I send you a nice jar of peanut butter." ~A fat kid in my school talking to my friend who has a fatal peanut allergy
"And I'll buy you some slim fast." ~His responce