The USB connection allows you to monitor the system audio device output separately from the other digital inputs (SPDIF/AES/Toslink) by a switch on the front panel of the DAC-1. As long as you have more than one digital out on your audio interface you could route a digital out as you have indicated, and monitor from a separate digital input into the DAC-1.
If you monitor via USB you are hearing the OS's system audio device. If you monitor via one of the other digital inputs you would simply route your master buss output to that digital out, feeding the appropriate input on the DAC-1. However you have to select the input source on the DAC-1 using the front panel toggle.
From their web site:
"here are four digital input connectors (coaxial, balanced XLR, optical, and USB). The coaxial, XLR, and optical digital inputs all accept professional (AES) and consumer (SPDIF) data formats at sample rates up to 192 kHz."
"The USB input is compatible with Windows ista/XP/2000 and Mac OS X, and does not require driver installation or system
configuration"