wow.. luckyli i own D2 , S9 , RE0 , fiio e5 too. , Personally i found the best settings by this procedure : plug re 0 to fiio 5 , do not plug in fiio 5 to d2 yet and increase the volume on amp to level that has no hiss or very little ( acceptable thanks to 64 ohm impedance ) Once u set it up , then connect to d2 and change volume to ur need. dont touch fiio5 volume anymore. good luck.
P.S. re0 can sound even better if u remove filters and yellow foams inside module canals. dont worry its no rocket science work. u will experience little nirvana. Soundstage + detail in mids/highs improved enormously. me personally i dont use fiio 5 with re0 . no needed. re0 sound just awesome straight from D2/S9 and honestly fiio 5 is low end amp.