XS2/SN2 Headphone Amp

Living in an English semi-detached house with poor sound insulation, the enjoyment of music at late hour is severely compromised. A good pair of headphones should be the solution!

I started reading up on how to choose headphones, and headphone impedance appears a critical issue. This makes me wonder about the ideal headphone impedance for the XS2/SN2 headphone amplifier -- it is my understanding that the two Naits share the same Class A headphone amplifier. 

Advice would be most appreciated -- many thanks!

Timo

P.S. I have no intention to go down the Headline route -- but maybe a Hugo in the future to beef up my ND5XS...