Yes, @soix, the U18 has galvanic isolation on its USB input, and so does the Micro DAC. As I mentioned earlier, the Micro DAC also allows the user to select either the I²S or the local clock, and I slightly prefer using the local clock.
The reason I'm asking now is because the return deadline for the U18 will have passed by the time the Rivo+ arrives.
Anyway, thank you for your response.