You could tell it you have 5.1 speakers and at the same time set "For stereo sources, only mix to 2.1", if you prefer your stereo sources to remain as such. There isn't really anything much in between stereo and 5.1, and LAV Audio would expand anything in between to 5.1 anyway.
Or of course just setup an external LAV Audio and enable the expansion option, until we handle this case in MC.