I just got a DAC1 USB, and have not changed any jumpers, and have so far only used headphones.  The manual says: For optimal L/R balance, the headphone gain jumpers should be set so that comfortable listening levels occur when the ‘Volume Control’ is set above the 10th detent.  Also:  Internal jumpers reduce the input to the HPA2 by 10 dB and place the Volume Control near 12 O’clock at a comfortable listening level with most headphones.

On my system, Windows Vista running Mediamonkey, with all volume levels set to max, a comfortable headphone listening level typically is with the DAC1 USB volume control set to ~ 9 O-clock.Is this the best configuration in terms of audio quality, or should I reduce the Windows volume control or the Mediamonkey volume control so the DAC1 USB can be set  ~ 10 to 12 O'clock?