review: I chose the NuForce over the Fiio E7 because the NuForce is a straightforward USB DAC/AMP, and therefore parts quality is concentrated solely on those functions instead of additional features like Bass EQ or display, etc.
http://www.amazon.com/NuForce-uDAC2-HP-Black-Headphone-24bit/dp/B00442KO0Q/ref=pd_cp_e_1