I have an Denon AVR-1600 receiver. Flipping through the manual I noticed a mode entitled " 6ch ext. in" . It goes on to say 'connect the output of the external DTS decoder etc. to this unit's 6ch ext in'. This brought up a question. What is this? When I play a DVD and turn it on to this mode, I get no sound. Do I have a DTS endcoder or did I not hook something up correctly? I currently watch DVD's using the "Dolby surround" mode. What is this 6channel mode?????????