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

Usually, but not always, setting the volume knob higher is better for sound quality as less attenuation is required.  My volume knob is always between 3 and 6 o’clock (i.e. fully turned up).

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.

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

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

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. 

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.