Hmmmm...Sorry what's the problem? Are you really unable to reach your listening volumes or do you simply need to turn the volume up a little more?
In the first case the solution is simple, you need a more powerful amplifier, in the second case there is no problem. In xlr the fosi has a gain of 25bd if I remember correctly, so your DAC can drive it at maximum power, if even at maximum this is not enough you simply need a more powerful amplifier
I'm starting to be very doubtful about this range of class D amps. So this 120wpc amp would be much less powerful than my class AB of 80wpc (Sansui AU 9500)?!
It's very annoying...