Mikward - from what I have read it is not difficult to repair the tweeter, not sure if the leads are soldered so may require that skill. Easiest/cheapest way may be to just remove the midrange/tweeter head and just bring the unit into a shop with the new tweeter parts. Maybe someone that has done it can chime in here.
B&W Service manuals can be downloaded. The pdf files go into lots of illustrated detail on how the speakers come apart
Scroll down near bottom and click on the Matrix 801 S3I have found the 801s to be very tunable with different cables, amps/ preamps, placement. I have been playing 15 IPS master tape dubs on them. I put on the lp and or cd after and it either sounds ok or not great. Its easy to identify which lps/cds have been engineered well based on the tape.
This thread got me curious so I did a search here at audiogon. Came across this ad. I am not connected to it in anyway Just thought it was interesting. Check out the mods on this 801.
Studio MonitorsCheers