Just a quick point to remember. Having a non-existant 6th channel results in the loss of a potentially significant amount of rear channel signal. You might find it well worth your while to explore adding a real 6th speaker even if the placement is not optimal. You could even go to a 7.1 setup by hooking the two rear centers in parallel to get a more diffused image (I haven't done this so I can't say if it results in a superior result).