When you changed it to "Dolby Digital" did it still show all five speakers and the LFE as being active? (That would confirm that it was a Dolby 5.1 source, in which case there may be a bug with the way the 990 is reporting the current processing mode...) Ooh! Are you running the latest firmware? If you are still running the firmware that you had back in the spring (3.07) rather than the newer firmware (3.11), pressing the surround mode button once actually changes the processing mode...