Samsung @ Pie Hi-Res sample frequencies require don't support DVC unfortunately ATM (this can change in the next ROM updates, something like that already happened in Android 7 days). You can get the same volume level by enabling "No Headroom Gain" option for Hi-Res Wired Headset/AUX output. This removes ~6dB headroom making volume the same as system player, but you loose headroom for equalizer (esp. bass) amplification.