It's funny you say that, that's exactly how I feel about the Arya V2.
In fact there's only a handful of headphones that I feel like is a must have in my collection. HD650 being one of them, and now the Arya.
Although, the Arya technically beats it in almost every way. There's just still something about the HD650. Maybe it's the sweet musicality in how it translates music or the warm chocolate mids? The only other headphone I must have is a pair of Grados for their timbre and acoustic/rock.
On another note, the EF400 isn't a very detailed DAC, but it's very musical sounding and the synergy with the Arya's is just amazing (as expected). I think I'm gonna keep it after all.