now for the nerdy definition, distortion in a speaker, is the loss of control of the voice coil. the voice coil is connect to the speaker cone and with electric current it moves back and forth rapidly. the different rates that it moves back and forth are your frecs IE why some bass songs sound really good on a sub and bass songs sound like poop, the frecs were matched perfectly for the power and size of the sub.
now were RMS comes in, RMS on the speaker is the amount of power needed to control the voice coil at all ranges of frecs the the speaker is designed for.
when you have less the required RMS you forgo control of the voice coil at peak performance. and the less RMS you have vs RMS needed controls how high you can turn it up befor it distorts
the more distortion the faster you blow the sub speaker ect.
%80 of the time after market head units and any good pair of aftermarked speakers are good/close enuff that it will get to loud befor it distorts. but if you have subs they can cover the disortion and you blow your speakers so its best to test them by highs and lows.
i have noticed that its the lower frecs were you start to lose control first.
IE trying to push to much bass at to high of a level thru a 6X9
