Hi Porsche, welcome!
I'll leave point 1 for others.
Issue 2: Is the music actually a 384Khz file? Or where you upsampling all your music to 384? If the first one, you may need to configure the sample rate to auto or something similar. If the second one, you should re-select the upsampling target. These instructions are missing from the DAC-10 manual, but can be found on the DAC-9 manual:
1. Power on NuPrime DAC-10, select USB input. The device will enumerate as a USB Audio device on your Mac.
2. In AudioMidiSetup, select the “NuPrimeDAC-10” device as your default Input and Output device.
3. The sample rate of the device can be selected using the dropdown menu.
Some names may differ.
Issue 3: Leave it a LL. This feature is intended for pairing with other types of amps which may require other voltages. STA-9 are suited for 2v on RCA and 4v on XLR, so that should be the right setting.
Hope this helps, best regards,