RPi USB is not very suitable to drive an USB DAC. This is because the RPi has a single on-the-go USB output. Some people do report feeding an USB DAC without problems, but a lot do. If your DAC has other inputs like SPDIF (coaxial as optical only goes to 24/96), you have better changes feeding it via an I2S SPDIF board on your RPi.
Anyone have a good answer before I buy a RPI 3, I already have older PI:s running volumio 2 with Hifiberry DIGI and DAC, but I need
an USB connection for this particular project.
By the way I run 24/192 files without problems over TOSlink.