Bruno, it is rather disturbing to note experiences such as yours which seem to indicate that the quality control isn't what it was in the past.
As to the mode to be used for the QS8s with 2-channel sources, my suggestion is to use DPLII rather than 5-channel stereo. The 5-channel stereo mode simply duplicates the left front in the left surround and likewise for the right side(a center speaker gets both the left and right front equally). While the ambience DPLII extracts from the front channels is of course very similar to the rest of the front channel material, it isn't identical. The ambience is slightly different in phase(because the reflections are delayed in time)and also is less prominent in the high frequencies and slightly lower in level(other properties of the reflected sound). Playing the original front channel sound in the surround may in a sense be more impressive since it's louder, but playing the extracted ambience instead is closer to reality.
