Hi,
Allo have released an update for Sparky kernel, this should resolve the USB issues you are experiencing.
To install it, please can you run the following commands:
wget https://github.com/sparkysbc/sparky_linux_images/raw/master/usbdac_mod_uImage.tar.bz2
tar -jxvf usbdac_mod_uImage.tar.bz2 -C /boot
reboot
Once completed, please re-run your tests with the USB DAC, and let us know the results.
If you experience any issues with the sound quality, or loss of device, please paste results of:
dmesg | grep usb