ZaenilleDefinitely fine!
It has no Hiss, do not change Frequency Response(very low out put impedance)
I recommend Modify High Gain 6.5->1
It's easy to apply just cut two resistors away!
Mine is x1, x2 gain, It works HD600, K701 to Er4 S and ETC
IEM to Full sized Headphones!
ZaenilleFor headphones from senn, it would work really well. There are lots of reviews about the pair. I recommend you to search more about it on google.
I'm not sure about Fidelio X2 since it has only 30 ohms, which means you can get enough volume without amp.
Hope this helps.