The nav unit in the sportech is made by Kenwood and you already know it is a DVD based system. I am not sure of the options available on that head unit for pre-outs but you can always just get a sound processor such as the Alpine PXA-H800 and then it will allow you to hook up subs, and other items as well depending on the unit.
Although for what you would pay for a signal processor I would just get a new navigation head unit from Kenwood/Pioneer or others and everything will be built in for you. If you were in a car with an integrated dash then the sound processor makes sense, but not in this case. All you need is a new trim frame around your stereo as the stock one is not standard double din size like the 2010 onward.