The USB Audio is preset as the priority output over S/PDIF and I2S, so theoretically if you have setup the system correctly, the USB Audio should always have output when the iDAP-6 starts to playback.
Can't figure out any problem based on your very simple description. Can you described your setup? When you said there is no output from iDAP-6 USB, can your DAC detect the iDAP-6 at all? Is there a LCD light displayed when your DAC locked up a USB Audio source correctly? or the DAC has stated there is no signal when you select USB in your DAC? Have you try to disconnect the USB cable and use other digital interface such as Coaxial or Optical? When you use iDAC-6 as source, did you connected to multiple DAC at the same time?
When you play a song on iDAP-6, the format is displayed correctly and the progress bar is moving as it should be?