I have never never witnessed, either personally or among the many friends with tube gear, a failure of tube gear that lead to damage of a speaker. Most tube failures end in distortion or no sound from the speaker. In a post above, someone mentioned problems with DC being sent to the speaker; this is impossible with most tube gear because an output transformer CANNOT output DC--DC in the primary would mean no output from the secondary because a fluctuating magnetic field (not DC) is required to induce a current in the secondary. The only possible DC from a tube amp would involve an OTL that does not employ a protection circuit or a blocking capacitor, and I have been told that that failure mode does not happen much either. I suppose that ANY high powered amp, tube or solid state, has the potential to do damage from a failure or operator error causing high level noise to be passed into the speaker.
On the other hand, I have been witness to a few solid state "failures" that did do in speaker drivers. Most often this was caused by operator error (e.g., accidentally pulling an interconnect from a source component causing a LOUD pop). I have also witnessed damage from just plain old abuse of playing at excessive volume--much more common with solid state gear because high power from solid state is "cheaper" and more common.
Owning tube gear might mean a bit more effort is required to maintain top working order--testing and replacing tubes, cleaning tube sockets--but, damage to speakers IS NOT one of those concerns. I would be MUCH more nervous if I owned high-powered solid state gear because of either failure or other conditions that can cause very loud noise, such as power outages/fluctuations (I've heard near heart stopping thumps from solid state gear when power flickered off and on because of crappy service in my area).