I don't know why your receiver manual said that. My DVD manual advised me that I need both the stereo RCA audio outputs connected AND either the RCA digital OR optical digital connected. Some DVDs don't send 5.1 digital so neither digital output is used. My Sony receiver had to be TOLD to use the optical inputs for 5.1 digital. Be sure yours is set to. It could be that your receiver has no way to select, and will just use whatever signal it gets.

Dave