The first thing to consider is are you happy with the current o/p frm ur headphones , if no then u will need to buy an amp .
I have a e5 and a udac as well , i had first purchased the UDAC for purpose of higher volume from my pc for ad700 , it did indeed provide me higher volume but i did not like the change in sound detailing it provided , now trying it on a Nuforce Udac i would say i am pretty happy with it , it has given a good volume boost and gives good sound detailing as well (by detailing i mean better beats , soundstaging and not to mention the bass in ad700 has gone up as well).So will be disposing of my E5 pretty soon.