How should I configure Hifiberry FAC+ Pro device settings?

I have a Raspberry Pi 3 Model B running DietPi (Linux 4.9.62-v7+) and a Hifiberry DAC+ Pro. The DAC is connected to a Rega Brio amplifier, which also has a turntable connected through its phono input. The amplifier does not have any input trim and I’ve observed that at the default settings, the Raspberry Pi+DAC combo with Roon is quite a bit louder than the turntable.

I’ve set default values for ALSA (i.e. all volume settings are 0dB). In the Roon Device Setup screen, I’m using Fixed Volume for the Volume Control Mode. What I’d like to do is to trim the volume down so that the levels are similar to the turntable. Is the best way to do this through ALSA mixer on the Raspberry Pi, and if so how should the Volume Control Mode in Roon be set? Or is it better to use the DSP Volume setting in Roon?