The speaker placement relative to the TV is not generally the reason for AV sync - the time that it takes sound to travel 18" is very small, and you'll be dealing with an adjustment of probably 1ms or 2ms. The reason for AV sync adjustment is video processing - it takes your TV (and the rest of your video signal path) time, sometimes 50ms to 100ms or more, to complete the various video processing tasks that are needed for a given source, and when the audio signal path is separate from that video path the audio doesn't get delayed automatically to compensate for that. When you have both the 990 on and the TV speakers on, you are hearing just how much audio delay your TV is automatically applying to compensate for the video processing that it is doing.