UHD3000 and Z9X : HT4 Volume output through USB is soft

When playing movie through HDMI video and out audio through USB to external DAC, the volume output from the player is very soft even when push the maximum on the payer. Even switching on Volume direct mode to get maximum output makes very little difference. Is this a bug or is there some calibration/adjusting that needs to be done to increase the volume output ? Appreciate some advice. Running FW 6.3.92_G.