That's an interesting question that I pondered in the past and from what I've read actually detered me from considering an iPad/iPod as a viable playback source for dedicated listening (and this pertains to the previous version of iPad, so I don't know if it still applies).
1. The iPad can only provide bus power to low-power USB devices (I think it's around 100 mA of curent draw, or less). Devices that require more power will either need to be self-powered (typically via a battery or AC adapter), or they will need to be connected via a
powered USB hub so extra costs associated there.

2. High speed and 24-bit devices are not supported.

3. Devices which do not conform to the USB audio class are also not supported. If a device supports both 24-bit and 16-bit operation, it should be switched to 16-bit mode before it is connected to the iPad.
4. Only sample rates up to 48 kHz are supported.

5. Asynchronous USB audio devices could experience periodic data loss when used with the iPad.

If it were me, I would take an old crappy Laptop with Windows Vista (or Windows 7) that I picked up on Craig's list for $50-$100, combined with an optional external USB harddrive $50-$80 (optional if the laptop doesn't have a big enough harddrive space to hold all your music) and an Asynch USB DAC over playback with an iPad (even over a DAC that was specifically made for an iPad).
But if there isn't any other means than using the iPad, just keep in mind that there are many low cost alternatives that will give you MUCH BETTER playback than the iPad as a direct source. I would just use it as a temporary means to listen to music, but would move onto something better in the nearest possible future for critical listening.
So ... I would not build a system around iPad playback as a source, I would not buy an amp or integrated amp around iPad playback as a source and especially wouldn't buy a DAC around iPad playback as a source, but rather buy components based on better playback in the future.
Anyways that's my .02 about the subject.