I'd be willing to bet that the 816 has better options for setting it up. I'm using the 516 at the moment and it has a feature for setting your room size (S, M, or L) and also your seating position (whether you're closer to your front speakers or the rear surrounds). Granted, these are manual settings. I doubt if the old 815 model included such settings. Go with the 816 and be happy. I'm quite sure the sound will amaze you.