My DAC is running in Class 1 mode already so it doesn't seem to be a driver issue.
Why not try the class 2 and an asio driver? .
I have read that USB Audio Class 1 standard supports 24 bits / 96 kHz max when using native mode USB Audio Class 1 drivers and is tied to USB 1 Full Speed = 12 MHz . A USB audio class 2 or a custom driver is needed to run 176/ 192 kHz and higher. USB Audio Class 2 additionally supports 32 bit.
Class 2 uses High Speed (480 MHz). This requires USB 2 or 3.
This might be even more important if different devices sharing the same USB-Hub.If your audio and your graphics card are on the same hub, the bandwidth required by the graphics might cause the audio to stutter.Anti-virus programs polling the internet with high priority might interrupt the audio too.
Some reading here re USB Troubleshooting at
http://thewelltemperedcomputer.com/HW/USB_Trouble.htm which in turn references:
USB audio trouble shooting - Lindemann
USB audio dropouts on XP - Computer Audio Asylum
A cheap, fast and effective way to circumvent USB IRQ issues - Computer Audio Asylum
Pops, clicks, crackles, dropouts, or distortion – M-Audio
Crazy problem: USB port provides bad sound, but only at low CPU usage