Matrix setting

The manual is singularly unhelpful - simply says that the rear speakers only work when you're playing surround sound movies. However, I was pleasantly surprised to discover that then you submit on online enquiry to Samsung (finding the link is a feat in itself) they're actually quite helpful.

As it turns out, the answer is indeed to change the Pro Logic II mode to "matrix". You then have to muck around and manually change the volume of the individual speakers to get any appreciable volume from the rear speakers (and still not as much as I'd like), but it's better than nought!