TV: Samsung LN52A650 52” 1080p 120Hz LCD
AV receiver: Onkyo TX-SR606
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You can't route the coaxial from the wall into the receiver - you'd be missing a vital component - the tuner. Your TV has the tuner built in, and that can't really be bypassed if you don't have some sort of cable box.
Your best option is to keep the coaxial into the TV and use the digital optical out from the TV to feed 5.1 audio to the receiver. Normally we tell folks that the digital output won't pass any 5.1 audio formats, but the lone exception to that rule is when you're using the TVs built-in tuner - which you are.
HDMI for broadcast/cable/satellite doesn't offer any performance improvement. No broadcast channels (cable, satellite, OTA, FiOS, etc.) offer anything more than 5.1 audio formats. Also, nothing other than Blu-Ray actually outputs 1080p (cable and satellite max out at 1080i).