The LINE INPUT on the SPD-SX PRO is a stereo connection.
When connecting an external audio player device such as an iPod, check the cable and confirm that both ends (connection end to the audio device, and 1/4" end for the SPD-SX PRO's LINE INPUT) are stereo. A mono connection on either or both ends of the cable will not produce stereo sound from the SPD-SX PRO's PHONES output or MAIN L/R outputs.