Hi Azrco - I run a similar set up (I think) for headphone listening from a MacBook Air for Spotify or ripped CDs. I have AKG K550s and Senn HD600 'phones. For the more efficient AKGs, the Dragonfly v1.2 alone is adequate. It will even drive the less efficient Senns. I do prefer using a headphone amp for either, though. My recollection is I checked before buying and the Schiit Asgard 2 that I purchased can handle the input voltage supplied by the Dragonfly. I plug the DF into the computer and use a mini pin to RCA splitter + IC cable from DF into the Asgard. I don't do anything to deactivate the DF's amp. I plug the 'phones into the Asgard. Mostly I leave the Dragonfly volume on the MBA at 100% (iTunes/BitPerfect volume also at 100%) and control listening volume via the Asgard.
In the same room, I have desktop AudioEngine A5 powered speakers. I plug a mini-pin equipped cable from the A5 amp into the Dragonfly. It sounds great.
So, my input to you is the Dragonfly will probably work with your proposed headphone amp but since I can't speak about your E12 amp specifically, look at the specs for it and see what they list for input voltage range vs what is cited as output from the Dragonfly. I do think a separate headphone amp will give you more flexibility down the road in terms of headphones you can use. It might even provide better sound.
Good luck.
In the same room, I have desktop AudioEngine A5 powered speakers. I plug a mini-pin equipped cable from the A5 amp into the Dragonfly. It sounds great.
So, my input to you is the Dragonfly will probably work with your proposed headphone amp but since I can't speak about your E12 amp specifically, look at the specs for it and see what they list for input voltage range vs what is cited as output from the Dragonfly. I do think a separate headphone amp will give you more flexibility down the road in terms of headphones you can use. It might even provide better sound.
Good luck.