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

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.

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).