Enable DSD bitstreaming, that way when you play any DSD files it'll automatically bitstream those type of files, while leaving PCM files as-is. You can't control volume or use any DSP while bitstreaming. Typically, volume has to be set at 100%, otherwise you might get just noise when attempting to bitstream DSD.
But... as far as I know, there are a few DACs that do support volume control while bitstreaming. For those DACs there is an option to enable volume while bitstreaming (right click on the speaker icon next to the volume slider, and select "Enable Volume When Bitstreaming").