I'm not in love with the headphone amp section on the X5, but the line-out is very solid imho. So you've already got a strong foundation there. I'd probably work on adding an amp rather than getting a replacement DAC.
Headphone Reviews and Discussion - Head-Fi.org
Townyj
Soooo as an owner of a Concero HP, how would you rate it against it? I love my HP. Actually... have you ever used the K812 with the Concero?
amigastar
Very nice Review project86 as always, the Audinst Hud mx1 still holds a special place in my heart since it was my entry into the hifi world.
mneveux
Do you think the headphone amp would pair well with HiFi Man HE-400I's?
Bansaku
Nice review, but where is the list/chart of specs? I want NUMBERS dagnabit! 
do you recommend a dedicated desktop amp if i already have a dac over this (I have fiio x5) ?
i am looking for a big, dynamic sound of a solid state amp.