[gharnden]

The low-level is for movies since the connection from the source through the amp is made digitally. The high-level is for music since the connection from the source (SACD) through the amp is made analogue with no option for bass management.

Having the connection high-level for music is why the speakers are set to full-range in the receiver to ensure that low frequency info is sent out through the speaker terminals and can be passed on to the sub.