For future reference, I noticed the other day that Pioneer has a 10" low profile sub that is only 3 inches deep. They're a little spendy, but a single one would add the lows needed to fill out four small speakers running off of the head unit. I also suspect it could be made to fit in the rear parcel shelf without seeming overly obnoxious. Couple it with a small single channel amp, and you might be on to something. You'll be well over $300, though.
http://www.crutchfield.com/S-ifY1RsEzlgw/App/Product/Item/Main.aspx?search=pioneer+shallow&i=130TSS2541