Unfortunately, symphonic-mpd does not support USB DACs.
I used to use the Amanero Combo 384 and S/PDIF.
I always thought that was the sound of my DAC, but one day when I started to think about replacing it, it occurred to me to remove the Combo384 and later use the I2S input directly from the Raspberry Pi2 to the remaining exposed I2S jacks.
At the time, I was using them in my DAC slave.
In other words, the clock source was a Raspberry Pi2 PLL. Even under these adverse conditions, RPi's I2S gave me a glimpse of the true performance of my DAC.
Those DDCs and receivers on my DAC did not seem to have any consideration for sound quality.
There are many choices of transports that support USB, whether Windows, Mac or Linux.
On the other hand, transports that can output I2S have been very limited, including products that run asynchronously, such as DDC and FIFO, and single board computers.
The lack of a transport that would fit my needs, and the fact that the I2S output of a single board computer can vary greatly in sound depending on software and settings, led me to build a crafted Linux called symphonic-mpd.
As for the output to the USB DAC, I haven't been able to test it here.
At the very least, you should have to set up your playback software to use the ALSA drivers.
Maybe you are missing a kernel module needed for the USB output. In that case, you'll need to change the kernel configuration and rebuild it.
There may be a problem with the compatibility of the Xenomai kernel and the USB driver.
You may also need to review the thread affinity and priority settings in /etc/rc.local.
As for the source of the kernel and driver, we are manually tuning hundreds of points by acquiring the profile data in the I2S output to improve performance.
These tweaks not only provide no benefit to the USB output, but in some cases may cause a performance degradation.
The symphonic-mpd specializes in I2S output and is not good at anything else.