Crap, Another headphone jack malfunction (first was in E17, tried to fix it and broke it :D).
Not sure if its poor headphone jack in Fiio equipment or if I am too rough on the amp.
A headphone that is high impedance and not sensitive will require a higher Vp-p which the E12 can deliver better than the X5. The E12 and E12A are no longer listed on their website, I was able to find the below link to get the below specs.
X5 : Vp-p = 8.2V
E12: Vp-p = 15V
E12a: Vp-p = 10.3
So the E12 is about 5.25 dBV higher than the X5. Whether this is important is up to how loud you listen, how much headroom you really need and how sensitive the headphone is.