high input and low input

High level (speaker level) input is not as stable, does not have as good frequency response and is prone to miss some of the lowest freqs.  Use the line level if the deck has RCA preouts, and use high level only as a last resort (usually with factory decks).  In fact, if using a factory deck, buy a separate high quality line out converter and leave the amp's high level converter alone.

http://www.bcae1.com/loc.htm for more info.