I think all you need the Camera Connection Kit. Lightning to USB. This would not work with some amps that takes too much power from the USB port.
If you use the analog output of the iPad you are using the DAC of it. It's better to feed digital to the NFB-11 so it can handle the conversion.
Other thing is to use 3.5 TRS to RCA input in the NFB, this way you skip the analog conversion but 100% is going to work to use the amp section.