You've done everything I would have suggested, Martin, but I would try the side surround connections for the rears, as often the 'back' connections refer to 7.1, but you only have 5.1 speakers. I think this is how I set up my newish AV amp, which is 7.1, but I only have a 5.1 set up, using the 'side' speakers as the 'rears'.