Welcome
@Ante Pan!
The Xonar U7 only has a digital volume control linked to your Operating System. So realistically, you wont be able to leave the volume on 100% all the time, because it could get too loud for your headphones/earphones/speakers and damage them.
There are exceptions to this, for example you have audio gear that is very difficult to drive and doesn't get very loud with the U7 at 100% to begin with, or you are plugging in powered speakers with their own volume nob (in which case you should set the volume of Windows to 100% and control the volume through the speakers themselves), or you are using an external amplifier with your U7 like I do, then you should max out the volume in the Operating System then control the volume directly from the external amplifier.
To get the best sound quality, the idea is that you try to max out the original source signal in the chain from your DAC
before it reaches your headphones/earphones/speakers without getting too loud, and you control or restrict the volume through other means like an external amplifier (if you use one). Powered speakers generally have their own built in amplifier, which is why they tend to have a volume nob too.
Setting the volume at 100% is not
always correct though, because besides potentially being too loud (without an external amplifier to restrict the volume), if you use a relatively weak DAC/Amp like the U7 with some
very hard to drive headphones, the phenomenon known as "clipping" could occur. When clipping occurs, it is generally pretty obvious to make out as it sounds like distorted, noisy peaks in the louder parts of your music. In this case, you have to lower the volume within your Operating System until the clipping no longer occurs and increase volume though other means like an external amplifier to compensate.
This is more of a tip, but in the U7's Audio Device properties, there should be an Amplifier Gain setting. For best results, make sure you leave it at +0.0 dB because there's really no point in lowering the power of the U7 unless your headphones or earphones are extremely easy to drive and you get some channel imbalance because you are forced to leave the volume % very low or else they get too loud.
Hope this helped!