Dolby and DTS both specify that rear channels for 5.1 are "sides" and not "backs." If I have 7.1 output, these files send the rears to the MS-defined back speakers when they should go to the sides. Maybe some people want it that way, so maybe it should be an option on the decoder to select which a person wants. Other options are simply too manual and would require changing settings for different files. Channel swap is one, and so is a custom matrix. If one could select this option on the decoder or a separate post-processor that would detect when channels=6 and just allow an overwrite of the channel mask, things would be simple.