As far as im aware the standard head unit in the Stilo has no options for preouts so you couldnt run an amp from it..........so wouldnt be able to run a sub. You can change the standard speakers, although from other threads ive seen it doesnt appear to be straight forward, and again you wouldnt be able to amp them up.
Im sure if im wrong there will be someone along to tell me so. Best option would be to change the head unit to an aftermarket one if your looking for big sounds