In TotalMix FX I have separated my audio sources into three types
- Audio from PC and applications without ASIO support to SW Playback AN1/2
- Audio from MusicBee (MusicPlayer with ASIO support) to SW Playback AN3/4
- Audio from Games to allow a different level than Youtube Videos to SW Playback AN5/6

For all (but one very spevial use case) I keep the volume level for SW Playbacks at 100% (0dBFS).

TM FX acts like a patch bay here, this audio I can route to different destinations.
In my case all goes to HW Output AES 1/2, where my ADI-2 Pro FS R BE is connected.

Besides quality and all other very nice unique features of the ADI-2 Pro FS R BE this device also acts as monitor controller to me. It allows me
a) to switch between Monitor and Phones by remapping this toggle function to one of the front keys
b) to keep the control over the final volume so that it doesn't can become too loud suddenly

An alternative to that is
- to use a passive volume knob
- to use passive attenuators to bring the volume into a range that is not too loud (see: https://forum.rme-audio.de/viewtopic.php?id=25399)

On the application side (MusicPlayer, DAW) I usually do not alter the volume to ensure there, that the audio signal comes out with highest quality / dynamic.

With instruments you need to look, either they have an analog output with a fixed level, then you simlply choose a proper reference level on the analog input not to overload it or they have a poti and then I would set it to 75-80% to avoid any distortion on the output and then look what reference level fits .. and then small adjustments on the volume knob of the instrument.