Fx audio dac x7 is excellent with pro 82, noticeably better than any amp of mines. I did not try O2/SMSL-idead, but O2/fiio q1 mkii. Until I found fx audio x7, pro 82 has trouble with big band music like orchestral piece and sounded disjointed sound. And it turned me back to th-900 or Nighthawk to get fuller sound. My intention for x7 was to get an improved sound with night hawks which has also problem of not natural sound even though it provides with the most harmonic density I ever found in music especially big band music. I notice that dynamic range increases significantly that means I can increase volume significantly without hurting my ears with proper fullness of the sound. Now my usual phone is pro 82 for the troubled big band music.