i have had a 2910 for about a year, & still don't fully understand what in the set up options can cause some of the options to be gray (unavailable). at times i have had trouble recently w/ analog out as i recently installed hdmi connection for 1st time. hdmi is straight to tv as my avm 30 lacks hdmi. how i fixed it was as non-technical as it gets, i went through the setup & changed different set up options until it worked again. last time it did it all i changed was the hdmi setup from 2 channel to multi & had sound via the analog connections. while i like the 2910, the setup operation can be a pain. my source direct has almost always been on, & if i remember correctly that does trigger some of the audio options from unavailable to available. also want to make sure that it's set to multi channel & that you have all the speakers activated in the 2910 - there may be a setup option where the rear speakers for example are set to none.
on other thing about "multi" channel sacd, even though labeled multi or 5.1 any particular sacd may not in fact use all of the channels, or it may not use all the channels on all the songs. compare diana krall's girl in the other room sacd which makes little use of the center channel for most of the songs (vocals are predominately from the front speakers) as opposed to her 01 or 02 sacd where the vocals are from the center channel. alison krauss & union station live - about all you get from the surrounds is clapping, & vocals are again from the fronts not the center. ray brown trio live at loa makes fantastic use of the surrounds, as opposed to just background effects.
i can't answer as to a rear channel(s) as i don't have one, but i would imagine that it is utilized differently on different sacds. i went through the same thing as i went without a center channel for a long time until i hooked up 2 center channels using a 2nd set of floor standers (if you have the option to do so this is a substantial improvement for multi channel music & movies) & thought that things were not working when in fact it was how the sacd was recorded &/or mixed. you may find out that it's just the sacd & not a problem w/ your system. hope it helps.