From my findings, the gains is directly effecting DAC output, and since it is a Current output, it can affect different Amplifier differently.
When I connect to my Desktop Amp, it was best to use Gain IV. I have had the most staging, layering, details than the rest
When I use directly, it is best to be on Gain III as I drive HD800S and have found the best compromises between Dynamic delivery and stagings
When I need it to drive In Ears such as IEM, I put it back to Gain I to have the best Dynamic and volume adjustment as possible.
Also when I need to use Gain II is where I want to see the battery from DAC/Analog to be as closely drained as possible with my 800S. Also has more staging but less dynamism
Now, onto the DAC volume. I found that If I switched on DSD Volume matching, it has Zero effects and particularly greyed out in MangoOS. Eventhough I am playing PCM at the time . It also doesn’t offer all the adjustable range that you could have had, but more like within 20%.
Also, DSD Filters do effect both PCM and DSD alike, and it is Best to be on High as I found better textures, extensions, imagines