If you have a 3.1 setup and you tell your surround processor or receiver that you don't have surrounds (in the 990, this would mean going to the speaker setup menu and selecting "NONE" rather than "LARGE" or "SMALL" for the surrounds), the processor will redirect discrete surround data from the surrounds into the fronts so that the information isn't lost entirely. Additionally, modes like Pro Logic II and NEO:6 will be much more limited in nature as they will not attempt to create any surround channels in the first place. (Also, the 990's 5 Stereo and 7 Stereo modes will not be available unless at least five speakers are enabled in the speaker setup menu...)