Just wanted to clarify, this unit, the DO100 Pro does NOT have this switch. Its not underneath, or anywhere on the case. It's also not mentioned anywhere in the manual, or website, so not totally sure where that image came from.
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I just leave it on 99 volume as I want maximum output (5v) to my headphone amp. I also assume that this delivers the best signal to noise ratio, but I don't believe it's been measured by amir.
If you truly wanted just 4v output, you'd have to lower the volume and measure the output until you hit 4v.