If my DAC has gain like my RME , I will experiment with the output voltage and set the volume to "0dB " on the DAC and use the preamp volume ,
For most RCA installs I had that DAC set to +5 dBu and it put out about 2 v to my Preamp. Set it too low and you invite noise by having to crank up the volume due to low gain. Set it to high and you can overload the preamps inputs.
If the DAC has no provision for adjustment, I set set the volume to 0db. If the volume does not read out in - dBu , like "-99 through 0" I will set it anywhere between "-10 and -3 "