Car Speakers

Hey guys, i have 4 kenwoods in my car, i beleive are rated at 125 max/40rms....i have an old 600 watt jensen amp (given to me) that was pushing a 12" sub, and working great. im planning on buying a new amp/sub hook up, so i tried to connect the jensen to the four speakers inside. i tested the amp to the two rear speakers, but it doesnt sound right. sometimes i hear a buzzing noise constantly as the rca is plugged in, and then sometimes the noise is gone, but the speakers begin to crack when i turn them up. im not sure of the wattage of the amp, but its 600 max 4 channel amp. does it not work with the speakers? or am i doing something wrong? also, if i can get the front speakers to work with the amp, ill connect two speakers plus the amp. thanks, and help.

First off the point of an amp is to run subs not mids. they can b ut they are not nice for the mids they can even blow them. the only thing that the speakers need is a quality deck that has min. of 50x4 so check that stuff out. enjoy my addvice