Sparkos based headphone amp

Gentlemen,

I'm about to build the amp for the 120 Oms headphones:
HD 560S | DIY-Audio-Heaven

I have 4 Sparkos Labs discrete single channel amplifiers which i'd like to utilize in there. Can they drive 120 Oms directly? Or, if there is the need for a buffer can it be BUF634A?

Assuming the Vout = 5Vrms and input = 2Vrms (0dB) the gain of the amp should be 2.5x, am I correct?

I'm new to the headphone systems, 'd like to gain some experience with this project. May be you can suggest well known DIY project.

Thanks in advance!