Well, it looks like there is nothing I can do to make the USB DAC function work, even though every other FiiO DAP has worked flawlessly, as well as a Questyle QP1R and the iFi Audio Micro iDSD Black, with my current Windows 10 laptop.
If I am to return it for a refund, I simply can't wait for FiiO to correct this bug. It's been on the market for about four months now, so it doesn't seem to be high on their list of priorities. It was either here or the FiiO forum where a FiiO representative said he tried two systems and it worked on one but didn't work on his office computer. This can't be the first time they have had a failure!?
I can't understand the hit and miss nature of the USB DAC function when the X7 works OK.
I could get another X7 for not much more money, but with the X7ii rumored to be coming out, I would end up regretting it.