I disagree with this. I have been using Tera with 1plus2 over the last year or so and it's the best SQ I have ever heard - without any additional amp.
I have also purchased the Ref 1 and am burning those in at the moment. Hoping they may take care of the occasional sibilance 1plus2 struggles with with some recordings (but only on handful of hot recordings).
By the way, in my opinion Tera is better than both AK120 and HM901 - don't know about AK240 as I have not had the chance to audition it.
Of course the trick with Tera is to get the right phones - meaning either Koss Porta Pro if you want open, or one of the Tralucent IEM's - nothing else I have tried gets even close to how great those pair with Tera.