Edwin, it isn't clear what "7 channel mode" you used. This usually means "7 channel stereo" which just duplicates the left front in the left surrounds and likewise for the right. It's mainly intended for two channel materials at parties, etc. to sort of get the same sound all over the room, but isn't really good for 5.1 movies.
The best mode in general is to apply DPLIIx Music(not Cinema)added to Dolby Digital. Although the full advantage of DPLIIx in creating stereo back surrounds can't be had with just one back speaker, DPLIIx will extract a mono back surround signal from the sides and direct it to the single back speaker.
