Check that your levels are within normal boundaries. Your amplifier will distort if more sound, or signal, is fed through it than it can process, causing a distortion in the sound you hear. Most good audio amplifiers have a signal indicator, usually two bars with green lights that rise and fall according to the strength of the audio signal. When the lights average about three quarters of the way up the bars, the level is perfect. If the level is higher, the lights at the top of the bars -- which are usually red -- will light up. This means the signal is too high and is likely to cause distortion. Reducing the volume will fix this problem.
Check your cables for kinks, fraying, cracks and breakages. Since your speaker cables carry the signal from amplifier to speaker, their quality will define the quality of your sound. With lower quality or damaged cables, you risk losing low end and peripheral frequencies, resulting in a flat and dull sound. Run the cables from your amplifier to your speakers in as straight or loose a line as possible. The cables are flexible, but bending them sharply for long periods of time can damage them and cause distortion.
Remove the covers from your speakers. Most speakers have a covering to protect the cones, which can be pulled off safely with little effort. Play some music at a normal listening level and look closely at the speaker cones for any tears or cracks. Run your finger very gently across the surface of the cone to feel for small tears that you may not be able to see. The cones vibrate in response to the amplifier's signal. Cracks, tears or holes will result in air passing through the cone, which creates distortion.
Turn down the bass level. Too much of the lower frequencies can overdrive smaller speaker systems, causing distortion. Some sound systems have a visual equalizer that, like the volume bars, shows the levels of different frequencies in the signal. If the bars on the left-hand side of your graphic equalizer are peaking (turning red), the bass need to be reduced. If the bars at the right-hand side are peaking, the treble knob needs to be turned down to prevent distortion.
Try using shorter cables. The farther a signal has to travel through a cable, the more it may degrade. Cables that are too long it can result in a low or distorted signal.