The back of my Comcast x1 box does not have these buttons you're referring to.What I am referring to is the small hdmi button on the upper right corner. You push that until the preselected hdmi 1-8 source comes up. The system does not connect automatically. Hope this helps.
Still can't figure this out and am annoyed. I really don't want to have to run a new HDMI and run direct from Comcast to my tv bypassing the receiver. Then deal with optical out. Would that improve picture quality in any way? If so maybe I'd consider the hassle which means taking the tv down which is never fun.