I hope I'm understanding this correctly, if so it definitely sounds like a source problem. I see you have the 703, I own the 702, so assuming much has not changed have you tried Menu / Advanced Setup / Preference / IntelliVolume and boost the input level? Maybe the TV has an output level adjustment as well?

What I would do is run the optical straight out from the cable box to the receiver and if you want audio on your TV then run some RCAs to the TV from the cable box, just a thought.