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Can't imagine there being a crossover problem, however it is possible. It is most likely the tweeter. I believe that the CM5s2 has a rear plate, remove that plate and find the lead that goes to the tweeter, Measure across the leads with the ohm rating on a meter, being an 8 ohm tweeter, you get a dc resistance measurement of around 6 to 7 ohms or so. Tweeter is good. If you don't get a reading, they you have a broken voice coil, if you get a shorted reading then you coil has been burnt. In either case, you would need to replace your diaphragm. Ask B&W if they sell a replacement diaphragm for the tweeter... Also, If I am correct, B&W uses electrolytic caps on this series, while you are repairing them, replace them with a decent polypropylene (even a cheap polyprop) and you will get a clear improvement on sound.
switch the tweeters between the speakers - still no sound then it's the crossover. If you get sound using the other tweeter, then it's the original tweeter that is bad. (or use a cheapo tweeter from Fry's or something if you don't want to risk your other working tweeter)
I am voting with others hear that it's the tweeter. a crossover failing and killing output to a tweeter is unlikely.
I recently had to fo this with my Revels and it was the original tweeter that was bad.