In https://www.fostexinternational.com/docs/archive_products/HP-A8C.shtml they say that the driver (dated 2013) is Win10 compliant, but in the PDF readme they don't mention Win10 is supported. Therefore I'm guessing that this is the first version of a Win10 driver. Seven years have passed and Windows is now very different from the initial release. It's almost a miracle the DAC works on your host.
The installation procedure is, as I suspected, like that of a USB printer and one should be very careful to not mess up, because if you do, there is no easy way to undo the damage.
I am pretty sure you were careful, but for posterity (and for people never reading Release Notes or READMEs) I have to reiterate: You should first install the driver making sure the DAC has never before been seen by the guest (better unplug it from the host). Install the driver in the guest and reboot. After reboot, plug the DAC to the host, and pass it to the guest so that it recognises the new hardware.
If you do mess up, you have to uninstall the drivers with the DAC unplugged and cross fingers. In such cases I also check manually the registry and .inf files for any leftovers, and use the USBDeview utility to cleanup the USB subsystem. Therefore, it is always a good idea to make a restore point (to the host AND the guest) before installing a USB device that has its own drivers.
PS. Reading some reviews - very nice (and very expensive) DAC. I am suspecting that the new iteration HP-A8mk2 may share the same USB bridge, have you tried to see if its drivers (dated 2017) work in your host?
