There is some stuff here that might help you.
http://www.polkaudio.com/car/library/ Good crossovers are a must for High quality sound! There is a very small chance that a 50 watt/ch amp is going to distort a 120 watt peak speaker.Here is how I tune my systems.Turn all controls(bass, treble, eq, bass boost, ect.) to flat. Turn the gains on the amp to 0.turn off sub if you have one. if possible fade the amp to just the front then turn the volume to 3/4 now slowly adjust the gain for the front speakers just to the point of distortion then back off just a hair.then put fader to center and bring in the rear in the same manner just enough to fill in the back.then bring in the sub if you have one.if you dont have a sub the crossover on the amp should not be used unless you have to.if the problem is still there after that check that all connections are quality and that the speakers are wired correctly. + to + and - to -.
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Kevin
00' Super Black SE 5 speed
superblackxterra.tripod.com