Try to swop the left speaker cable with the right speaker cable first.. Maybe it's the speaker that's having the issue.
"Master reset of the Onkyo" done too?
"Master Reset" can be Handy to clear up niggling issues...but all settings will revert to factory settings..
My 820 had this issue too a couple years ago
Any luck with the call to Hwee Seng, buddy?
Did you manage to get it repaired? Roughly how much are the repairs?