1) Upgrade YouTube to 2.10.17, same problem.
2) Uninstall 2.10.17 and install YouTube Vanced, A10 USB Output OK, but no 4K.
3) Uninstall YouTube Vanced and install back 2.10.17, same problem.
4) Install latest Chrome, start YouTube within Chrome platform, A10 USB Output OK and got 4K.
The A10 USB output to Denafrips Terminator USB input, sound quality is better than Optical input. I am satisfied with this setup.