The differences between the sig 805 and the 805S are large, yet the sound difference is not that large.
805S uses a different woofer, tweeter, cabinet, wiring, and crossover than the sig 805.
The above could also be said about the difference between the N805 and the sig 805.
I firmly believe that the sound of the sig 805 vs. the 805S is strictly a preference, they are that close. I love the finish of sig, and prefer it slightly to the 805S in terms of sound quality. Having said that, the 805S is obviously the better value at $1000 less retail than the sig.
I also believe that the sig 805 really needs to be heard in your own listening room to appreciate them. Same with any speaker, really.