System : 

TV - KD-65X8509C
Soundbar - HT-ST9

I have recently purchased this set up, but am experiencing a problem. I presume it is a setting issue rather than a hardware one.

The devices that are connected via the HDMI ports have crystal clear sound. However, the device which is connected direct to the TV and so is using the digital optical out to provide a signal to the soundbar. This is experiencing sound distortion on what happens to be the deeper sounds.

Help please.