Line Level Converters are used to convert the level from high level (speaker level) to low level - the same level RCA cables operate at.
This can allow you to keep your existing head unit, by connecting a line level converter to the high level stereo outputs, which are then converted to a low level stereo signal, suitable for plugging into an external amplifier - to drive an additional subwoofer or parcel shelf speakers.
The speaker level to RCA level conversion retains almost all of the audio's quality and there is absolutely no noticable quality deteriation. As testiment to this, our customers have used these products to convert the stereo's signal to RCA leads, and then used an amp to drive the door speakers - achieving a very high quality sound at louder volumes where their stock stereo performed badly.
Where possible, try to get an adjustable line level converter - that way you can easily adjust the signal strength passed on down the RCA cable. You'll only have to do this properly once, and once done your signal will be matched to the output of your amp and all of the speakers output should be well balanced.