I don't have 800D's but recently bought a pair of Signature 800's. I replaced a pair of N801's. Both are great speakers, and the S800's actually pass the WAF.
I have never found the Nautilus or Signature tweeters to be harsh (the S800 has a different tweeter than the Nautilus per B&W's engineers). I use Bryston 7B ST's with a Musical Fidelity A308CR preamp.
The S800's image very well and dissappear. They seem to throw a higher image than the N801's, maybe due to their height. They are not overly critical of room placement, but toe in made a big difference in imaging. They are also really nice looking speakers. I highly recommend them.