If you are getting distortion you are creating heat which is what fries voice coils. It is quite hard to physically "break" a speaker with excess power, but not so difficult to fry a voice coil with unclean power. With that being said, matching wattage of speakers to wattage of amplification is not a great method of determining what speakers you need. It COULD solve your problem, but it might not. If the higher wattage speakers are less efficient, as in they have a lower SPL rating (this is measured by applying 1W at 1M and measuring the sound pressure level), you could actually end up with less sound even though they can handle more power. The head unit will most likely distort when cranked to high volumes regardless of the speaker attached to it. If you want more volume than the current speakers can handle, my advice would be to find a speaker with a higher RMS rating AND a higher efficiency. A typical 2 way speaker will have an efficiency rating of around 90. If your current speakers have an 88dB rating for example, you could actually replace them with a speaker that has a 91dB rating with the same power rating and get much more volume. So, to make this clear as mud, look for speakers that will handle more power AND that have a high efficiency rating.