Sound difference between High\Low Gain Setting when volume matched?

I would guess that the sound quality is the same.

you choose the gain acording to the headphones or speakers ( easy or not to drive ) used and the output of the source.

I’d suggest trying the lower output first to see if it’s sufficient to drive the headphones.  I slightly prefer the lower output setting on my Singxer SA-1 with my Hifiman Arya Stealth ‘phones.  Using a higher gain setting with sensitive headphones can result in unwanted noise or hiss. 

The lower output gives you more volume control.  Which is fine.

Depends a little on the design. AND if you can hear/feel a difference.

My dealer recommended using the volume pot between 10 and 2 o’clock and then using gain to add/reduce volume to the desired level. That way your adjustment swings aren’t too wide or too narrow.