Mike, a balanced input acts on a "differential" basis, i.e. it subtracts the signal on one conductor from the signal on the other. The result is that any signal that was exactly equal on both conductors would be completely cancelled. When outside interference hits two very closely spaced(often twisted)conductors the noise signal is nearly the same in both conductors and the noise is therefore almost completely cancelled in the balanced input. The music signals themselves are positive and negative on the two conductors and subtracting a negative is the same as adding it to the positive, so the desired music signal isn't cancelled, while the unwanted noise is. So, the balanced input has an inherent resistance to interference, even if the connecting cable isn't shielded. This is often preferred in professional applications involving long runs near sources of interference, so the XLR connectors to the balanced inputs are used instead of RCA plugs. As a practical matter in home use the usual shielded coax with RCAs to the unbalanced inputs has no significant problem with interference and the balanced inputs wouldn't bring any real benefit.
