We actually don’t do any processing in Roon related to volume. @brian discussed things in more depth here, but we’re simply sending volume commands to the driver.
The fixed volume setting simply sets the volume to 100% and hides the slider. It’s designed for cases where, for example, telling your DAC to play back at less than 100% might had adverse affects, like no longer playing bit perfect.
I think running with fixed volume off should work for you, but let me know if I’m mistaken. Thanks @loeribas!