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
Jolida has a nice DAC that is tubed based (12AX7) that I love. I have replaced the stock tubes with Telefunken smooth plate 12AX7s and LOVE the huge soundstage that was not present with the stock tubes. you mentions other DACs and I've heard the arcam, but have not heard the others so I can't comment on them. I actually like the Jolida better..and it won't cost you a small fortune. I got mine from Walter at Underwood Hifi...he's great. But's just my opinion.
Thanks I will check out the Jolida.