I just replaced the Nautilus Tweeter on an 803. I ordered the Tweeter from B&W USA, it was $98-delivered. It's very easy to replace. Just turn the front part of the Tweeter Housing counter-clockwise (it's treaded) and it will come loose, then gently pull it forward and disconnect (unplug) the red (1) & black (1) wires. I was surprised at how inexpensive and easy it was to replace. The cone on my Tweeter was also pushed-in, by a curious child, of course.
