The 970 will behave basically the same as the 990. It will remember your preferences for different source types, so if you select Stereo and Dolby Digital for two channel sources (PCM stereo and Dolby 2.0, respectively) it will remember that even if you select something else entirely for Dolby Digital 5.1. Or, you can select 5 Stereo for PCM stereo, Pro Logic II for Dolby 2.0, and of course Dolby Digital for Dolby 5.1 - and the 970 will remember these choices for this specific input and revert to them when these types of inputs are received in the future.

Something that folks sometimes don't recognize: receivers and processors must automatically recognize and decode Dolby Digital and DTS sources using DD and DTS. If they don't, you don't hear anything. This is an automatic detection and adjustment process that happens every time a digital signal is first received by the processor. These days, however, we also have additional surround processing modes that can be used in conjunction with DD and DTS as well as with pure stereo sources (most notably Pro Logic II/IIx and NEO:6). When we start mixing in these additional modes, it's common for folks to get pretty confused.