http://www.faqs.org/faqs/car-audio/part2/ is where the line driver answer is....
"A line driver is a device that amplifies a signal, such as the low-level signal output from a head unit. Line drivers are made to amplify the line level signal to as much as 10 volts or higher. This, of course, is useless unless the receiving end can handle 10 volts as input. To solve this problem, there are line receivers which bring the line level voltage down from 10 volts or more to about 1 volt. Usually, the line driver and receiver are placed as close to the sending signal source and destination as possible, to minimize noise pick up. The automobile is an inherently noisy electrical environment. So RCA cables may pick up noise as it makes its way to the amplifier. Note that noise here refers to the induced noise, not ground loop noise such as engine whine. A simple way to fight against this noise is to make the signal level carried in the RCA cable very high, thus increasing the signal's resistance to induced noise and resulting in a higher signal to noise ratio at the destination of the RCA cable. Most head units produce a fairly low output voltage (< 1.5 V), although recently high end head units advertise 4 volt or higher output, and won't usually need a line driver. The line driver will increase dynamic range in certain cases where excessive noise is masking the lower level signals. However, a line driver will not increase the dynamic range when used in a system with little noise to begin with. There is some truth to the claim that a line driver will let you play your stereo louder since there are cases where the amplifier still doesn't play at its full potential even when its gain is turned all the way up and the volume on the head unit is maxed out. Adding a line driver here will allow you to turn down the gain on the amp while using a lower volume setting on the head unit. But before you jump in with both feet, remember that all electronics has their own inherent noise. Thus if you don't have a serious case of induced noise, a line driver will do little good since it might add enough noise to offset what little noise it "takes away." The line driver is a patch to the noise problem rather than a fix so it is still not the ultimate solution. My personal experience has shown to ME that a properly installed system with none-malfunctioning components will have little noise, even if you use low grade components such as those made by the less desirable manufactures. Also, a lot of crossovers and EQ units have rather high low-level output signals. Some times as high as 8 volts. So be sure to take this into consideration"
