according to this: https://www.hifiberry.com/guides/mixer-settings/ you should keep the playback boost off and reduce the Digital Volume slightly to about 96%.
Anyway I would recommend to directly connect the squeezelite volume control to the hardware control of your DAC. With the standard setting you have a software volume control and a fixed hardware volume on your DAC – but this doesn’t improve audioquality.
You can connect the volume control for squeezelite to your DAC by adding
-o hw:0 -V Digital
to the command line options of Squeezelite in the advanced settings of the audioplay tab.