I have an Onkyo TX-600 receiver connected to a Comcast-Motorola HDTV cable box via an optical cable. When I change channels from an analog channel to a digital channel (like HBO) and the receiver detects the switch from PL2 to Dolby digital, I get a pop through my center channel speaker. Comcast came in and replaced the box and it still happens so they say it's the receiver. I sent the receiver to repair and the guy there thinks it's the cable box. Any one have a similar experience or advice? Thanks