Why are my Speakers Distorting?

I currently own a kenwood indash that produces 50 watts peak. I have 2 6.5 kicker door speaker with the component tweeters that max at 130 watts and in the deck i own infiniti kappa serious 6x9 that max at 110 watts. What i would like to no is why do my speakers gradually distort after playing them at loud for at least 30 min or depending on how loud the speakers are played the shorter time frame before they distort?? Is it because i need to connect a small 80 watt amp to them so they can obtain more power or is it because i need a new indash?? I currently have a pioneer indash i can swapt it with but it comes with less outlets for subs. Can someone help and tell me why they are distorting or getting "weak".??