If they are still under warranty just say the driver blew, especially if you can prove that your amp could in no way overpower them. (And if you have no proof that someone actually damaged the driver...)
Sounds like it was a defective tweeter from the beginning those things are fairly resistent (unless someone actually forcefully punctures it).
Most B&W dealers can get a replacement driver and with auth from home base can just replace the affected driver, no cost to you.
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