i just bought polk speakers (rti8, csi5, and fxi3) and my speakers are connected to the harman cardon avr240. i have a question about wiring. when i use digital coax to connect my dvd to the ampli, i am limited in the "choice of sound" - i can only choose "dolby". whereas if i use composite (rca) cables, i have more choices like pro-logic, dts etc... why is that?