At first, my disappointment of Scarlett 2i4 is a worse recording of line level signal.
What about playback with 300 Ohm headphones, like a Sennheiser HD 600, I think:
The Xonar Essence STX is simply the best for it price even without amplifier, but it is internal.
The Xonar Essence One is exelent, but for that price may be better use a receiver/DAC with S/PDIF or HDMI input.
I use (before and now) the Creative Sound Blaster X-Fi HD USB card ( http://borisgermanov.blogspot.ru/2013/11/creative-sound-blaster-x-fi-hd-sound.html ) with Corda Arietta Amplifier ( http://borisgermanov.blogspot.ru/2012/05/meier-audio-corda-arietta-headphone.html ).
It sounds little better then 2i4 without amp. Both 2i4 and X-Fi without amp with HD600 are the same. That is for me. I'm not a cool audiophile - I can't hear above 16kHz :-(.
I think, the Scarlet is good choice for using active monitors with long balanced cables (or for recording of Hi-Z sources and microphones without analog mixer).
I know, that this words doesn't help to any choice, but if I be able to find an any RMAA tests of the scarlett 2i4 I was not buy it...