Digital gain vs analog gain

Not sure I follow you.  What do you mean by digital gain?  Digital sound level is set on the recording.  If you mean that your DAC has a gain on it, that it the gain on the analog side of your DAC and is often used without a preamp.  As for which one to turn up, I'd experiment.  Set my DAC volume at low, med and high for a day each.  You may find it is a way to control harshness.

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 " 

Understood. When playing streaming music from my iPhone I can control the final volume of listening by changing the output level of the app (which I assume is digital) or changing the analog output level on my Shiit pre-amp to my analog amp or change the amp attenuation.

I’ve running my old Crown at 100% but am wondering which of the other gain sources is best to maintain clarity of signal. As I understand things excessive gain in the digital realm creates clipping and excessive analog gain creates distortion. How can I avoid these pitfall?

…aaand, knowing just enough to be dangerous, it seems that too little gain can hurt signal to noise rstio?