One of the best reasons for connecting over optical is you dismiss any ground loop and hum. You did that so it's possible the receiver has failed. Does a new receiver work right? Did this setup ever work right?
I have an older Kenwood receiver (10 years) and it is connected to my TV via a Fiber Optic cable, problem is the sound is so low I have to turn it almost full, which then produces a hum. Other connections are a Bluray (HDMI), PC (HDMI/DVI), and Rogers digital box composite connections (to get HDMI I have to pay 12 bucks a month so that isn't going to happen). I don't think this would affect the audio though.
Also, I have two fronts a center and two rears connected to the receiver, plus a subwoofer.
Any suggestions would be helpful.