I just finished my review of the relatively new Audinst USB DAC & headphone amplifier. If you are considering the Fiio E17 you might want to check this out. The HUD-Mini is a bit cheaper than the last model (HUD-MX1) - between £80-100. I tried a couple of different op-amps here and even include some pics of the circuit board and chips.
Enjoy and have a great rest of the weekend everyone!