Speakers have thermal limits and phyical limits. The thermal portion deals with the voice coil. While the suspension (spider and surround) manage the physical portion. If you have 42V on a 4 ohm @ 1k on a sub, you will smoke the voice coil, but the suspension will never flex. If you have that same load at 20hz the suspension will break first.
You see by the speaker moving in and out, it cools itself. So it has to work within cerain frequencies. So the over load would need to be considered based on the frequency being played since the speaker's impedance changes with frequency too. The power rating listed is normally just a point of reference. Some are modest in rating others are a bit optimistic.
If you were over powering, you could turn down your amps level so that the system was better matched with other components. If the amp is bridged, run the speakers in stereo (cuts the power to them) Or I suppose you could Series them if you did have them in parallel, again this is just limiting the amp's output. The passive crossover is most often used for component sets.