Hi, sorry for slow reply. For some reason this post was not automatically approved by the system.
The output is "automatic" and depend ont he impedance of the connected amp or headphones. For lower impedances like 30ohm, the current that flows through is much bigger than for higher impedances, say 50Kohm. This makes that Voltage go down for lower impedances.
This is generally nothing to worry about, and is just to inform that if you have 30ohm headphones, the dac will output max 1Vrms and not 2.2Vrms.
..but how are these achieved. Is it a matter of operating computer on full volume if you wish to connect to hifi amp but for headphone use you lower the volume as required? Or level set automatically when conencted to amp?