The 5 Stereo mode is fairly straightforward: The left channel is copied to the left surround, the right channel is copied to the right surround, and the left and right are summed to produce a center channel signal. You are likely to encounter an increase in bass as well as an overall increase in sound power level when switching between Stereo and 5 Stereo without adjusting the volume - this is because the copying process is done without reducing levels so that the different channels all operate at the same level to produce a greater combined volume, plus the fact that the bass management is now steering data from five channels to the sub instead of two.