I guess I'll make this my first proper post here:
I had the same dilemma a few years ago. Do I up the gain or do I up the volume on my amp? I found all sorts of conflicting info online. From my experience of Sat-coms it's best to get the cleanest signal into the final stage.
Any way what I decided to do is select the stages's gain somewhere in the middle (forgot which setting exactly) and also set the volume of my amp to something reasonable and made note of where the knob ponited
Next I played the pink noise track of the hifi news test record and using a dB meter I recorded my base point of amplitude. I think it was 85 dB.
Next with nothing playing I would set the volume of the amp to the 12 o'clock position and go through each gain setting making note of how much dB of noise floor I was getting.
It's a bit complicated but using the 85dB reference point I was able to go back and forth adjusting the stage gain and amp volume to find the best signal to noise ratio.
In my case like I predicted it was better to go very low gain and turn the amp up a bit. There's so many variables in our setups, cartridges, amps, mains, cabling ect what was the best combination for me might be a disaster for someone else! Fast forward to today I've gone to great lengths to get clean mains. My new amp and phono stage are both super quiet, it's almost impossible to tell the difference other than how loud each setting is.