It appears you haven't recieved very good responses yet. The original nautilus series was made availabe in 1997. It was a drastic change over the previous matrix series. It served B&W well for many years. The next iteration (late 2004) used an "S" in the model name. The "S" had nothing to do with the "Signature" line. The signature line only offered the 800, 805, and the center. The "S" came after the signature and included the entire 800 series in the line.
The tweeter from the signature line was shared in the newer "S" line, except for the 803D, 802D, 801D, and 800D. Those carried the diamond tweeter represented by the D in the name. I've owned the Nautilus 805, Signature 805, and the 805S. Based on my experience, the signature 805 was a significant improvement over the standard N805. The 805S was much closer to the signature sound (some say even better).
I also auditioned the N804 on many occassions and owned the 804S for a short time. In my opinion, the "S" line is more balanced, offers better bass, is smoother in the upper end and is just more enjoyable to listen to. There is a price difference on the used market between the N804 and the 804S, but given the choice, it'd be a no brainer for me. I'd take the 804S.
Hope this helps.
