Best DAC for iPad newest generation

I'm interested in a new DAC to use with Apple Devices. In order of importance iPad. iPhone, and iPod. I would like something that will hands down beat my NuForce iCaniDo. I do not require a headphone amp. Unit must have great DAC and clean audio stage. USB input (digital)and RCA output(analog). I'm considering Schiit Modi, arcam drDoc, and Schiit BiFrost. I would like some input on how these 4 devices compare, and suggestions on products I should consider too.

Thanks in Advance
Aaron