I have the Logitech Z 680 And I thought I had a problem with my Speakers. I called up Logitech and they sent me a new set off speakers Logitech Z 5500. I found out when I disconected the cables from the main module and reconected them every thing worked just find, Now I had two good working speaker systems. I have that same problem with the module every once in a while with both sets off speakers. I found out if the cables from the back off the module gets losse I have that problem.
And that is on both speaker systems.
Your problem could be different, but somthing to think about.

Good Luck
Wendell H