A word of caution (you may know this but just for the general reader's awareness) : Most modules using the S91 effect were not written with a surround setup in mind. Instead, they exploit the fact that in a stereo setup, it simply inverts one of the two stereo channels, so it's used as a cheap way to create fake stereo samples. Playing such modules in quad surround mode means that often you will just get random notes playing from the back channels for no apparent reason.