You'll need 3 of these (or simular): http://www.amazon.com/Male-3-5mm-Stereo-Female-Y-Cable/dp/B000I23TTE
Connect the actual speakers to your sub as normal.
Connect 1of the y-cables rca jacks to the denon Front L & R speaker out. Connect the speakers Front mini jack to this, the one from the sub.
Connect another to Denon Rear L & R speaker out. Connect the Speakers Rear mini jack to this
Connect the last one to Denons Sub and center which should connect to the sub & center mini plug. Please note that we have no way to tell if the rca's for the sub are on correctly. start off at a low volume on both the tv and the speakers and see if center and sub are working correctly, If not the swap the two rca jacks on the back of the denon.