See if you can get the service manual from B&W. They have them for the discontinued models on their website.
If the crossover is typical of most B&W designs, there is a single coupling cap connecting the tweeter to the input. There may also be a resistor in series with the coupling cap (but not always). So either one of these may be bad. I would temporarily jumper (short out) these two components to see if you have high frequencies, but do it at low volumes! It would be strange that BOTH speakers would have the same defect, however. You might also check the wiring to be sure everything is connected as it should be, particularly the wire connectors to the crossover boards
If the crossover is typical of most B&W designs, there is a single coupling cap connecting the tweeter to the input. There may also be a resistor in series with the coupling cap (but not always). So either one of these may be bad. I would temporarily jumper (short out) these two components to see if you have high frequencies, but do it at low volumes! It would be strange that BOTH speakers would have the same defect, however. You might also check the wiring to be sure everything is connected as it should be, particularly the wire connectors to the crossover boards