Hi ktm,
you can increase the volume by using the command "alsamixer" on console.
SSH to your Pi or use a local keyboard and monitor.
Execute
alsamixer
select your USB dac (F6)
and change volume.
- If you are not familiar with SSH read this for Windows or this for Linux/MacOS
The default login for RuneAudio is "root" with the password "rune" (without quotes).
Frank