High-gain vs Low-gain Headphone Amp

I am using Meier audio Corda 3move portable headphone amplifier, lately bought a Sennheiser HD600.
I wonder should I set the headphone amp at a Low-gain or High-gain , to get an optimal sonic reproduction?

BTW, whats the sonic differences between the two setting (High-gain vs Low-gain)?

I am anxiously looking forward for your opinions and suggestions and thanks in advance
The main difference is the position of the volume knob.
The only person who can decide which is better: low or high output.. is YOU listening to the earphones both ways and YOU deciding: "I like this position better".
That is it.
The theoretical tradeoffs are more dynamic vs lower noise floor. All dependent on the way the volume control responds at different settings.
No one else can decide for you what you would like better.

For MY headamp a Rudistor RPX333mk2 and my headphones, Sennheiser HD800 i like the lower level output better.
THANKS Elizabeth for your opinions and suggestions.
I did follow your suggestions and confer your opinions that I like the Lower level output better with my HD600 on listening to classical musics.You are RIGHT:all I have to do is just to increase the volume control little -by-little to find out "my optimal" sound