I recently bought a 809 after returning a 609 which I did not think was any better than the amp it replaced. The 809 is streets ahead of the 609!
I did not try the 709 vs 809 but I was told it was a lot better than 709, the 809 is a lot bigger and heavier, so it would surggest there is more lot more inside than just upgraded picture processing and is only £100 more.