One of the biggest differences between the 805 and 806 is the weight. The 805 weighs around 51lbs. The 806 weighs 37.5lbs. That's almost 14lbs. which is astronomical!
The transformer in the previous generation 805 was toroidal, while the one in the 806 is not. There may or may not be a real world difference in the two, but note the new 876 continues to weigh around 51lbs.
It seams to be Onkyo is trying to save a few dollars on the 806, while separating the 876 and 906 from the pack.
Is it possible the 805 is that "needle in the haystack" and "best kept secret" in audio, and gone the way of the dinosaur in the sub $600 price range?
A man meets a genie. The genie tells him he can ask for whatever he wants, but his mother-in-law gets double of what he gets. The man says give me a million dollars and beat me till I'm half dead.