I've been lurking for a while now, so I thought it was the right moment to register and give you my 2 cents.
Unfortunately you can't override your iDevice's DAC with some accessories like the Fiio L3 nor the DAC/AMP combo you suggested is capable to manage the digital signal coming out of an iPad... For instance, all LOD accessories are used to bypass the iPod/iPhone/iPad internal amp and there's no way to get some digital code through the analog stereo 3.5mm line out of the Fiio L3.
To get rid of the internal DAC of your iPad you need some particular DACs and they are quite pricey.
Just to name a few, you can try the well known iStreamer
(no batteries, so no portable at all), the AlgoRhythm Solo
(which has no onboard amp) or the really really expensive Fostex HP-P1
imho you should go for an inexpensive amplifier like Fiio E6 or the awesome Fiio E11 (especially if you are into high impedance/hard to drive phones
) and a nice LOD of your choice (like L3 or L9): the simple setup PMP + AMP will give you room to improve your portable rig with a powerful DAC like the AlgoRhythm Solo I cited above, if you'll ever need one.