hairy_hen said:
{a lot of sensible stuff about mono sound}
Thanks h_h, that's exactly what I was thinking but I wasn't sure of the facts. While searching at various home cinema forums I found this:
which suggests that although mono should come from the centre speaker, some people prefer to use the front left & right. But I also saw this thread where someone who describes himself as an acoustical engineer says that this can cause unwanted effects.