There is MORE danger to a speaker from an UNDER powered amplifier than the other way round. This is because an under powered amplifier will CLIP and destroy a speaker if the amp does not have any DC protection circuit. You can obviously destroy a speaker from VASTLY overpowering it but your ears will catch that long before that happens and will force you to turn the volume down whereas during clipping, by the time your ears hear it, it may be too late.
A good rule of thumb... Your amp should be probably at least twice as powerful as your speaker rating to avoid clipping.
A good rule of thumb... Your amp should be probably at least twice as powerful as your speaker rating to avoid clipping.