Hi--I have a Pioneer VSX-LX102 receiver and I am trying to connect my Sennheiser HDR175 headpones. I have connected the headphone cable to the Optical (tv) output but get no sound to headphones. What additional steps are needed within the receiver to get audio? I now have the headphones hooked up to the TV and it works fine after shutting off the receiver but would like to upgrade the quality by connecting to the receiver.