I think one of the major differences to consider is the 809 has Audyssey XT whereas the 818 has XT32. Plus am sure some other features, one other significant one I can think of is the ability to have a true electronic crossover for amplifying separate tweeter/sub-mid whether you're a speaker builder or otherwise, that's a pretty unique feature at this pricepoint along with the XT32.
"I realize that somebody playing free music isn't as commercial as a hamburger stand. But is it because you can eat a hamburger and hold it in your hand and you can't do that with music? Is it too free to control?" - Don Van Vliet (aka Captain Beefheart) discussing commercial success in the music biz