Once I had all three of my rear channels quit working. They just would NOT play a thing when I could plainly see all 6.1 speakers SHOULD be playing. I don't recall if they worked in the test loop or not. What fixed it was when I changed discs to check if it was the media for some reason. I didn't think it was as I had played it MANY times before (4 year old in the house, disc was Finding Nemo... you do the math), but I wanted to rule it out. Put in another DD disc and same problem. Tried a DTS track, instant life! Switched back to Nemo, problem was gone. Hasn't happened since. But that was also right around the five year mark.

Don't know if that helps, but it certainly can't hurt.