I can't compare the 976 to the 990 SQ, and I'm not a professional reviewer, but I can tell there is an improved SQ from the 975 (I used to have in the same system) to the 976. It sounds clearer to me with streaming as it did with the 975. Of course, I used the auto set up mic to balance the system with the 976, whereas, I did it manually with the 975. I did notice channels on one side of my room had the levels set lower than channels on the other side. It could be that one side has a long wall and the other a set of three windows with curtains/blinds (constructive vs destructive). I guess I need room treatments for that one wall.
With the 975 I used an spl meter to match all the levels equal. Sounded good at the time, but now it is a noticeable difference.