Thanks for the video link, I'd seen it a while back and my frustration was that I followed what I thought was the same set up as him yet was getting inconsistent results.
I'm now pleased to say that I have fixed the issue! I am now able to record through my amplify via USB into my iPad using GarageBand and the results are amazing. Sparklingly clear stereo recordings. The problem I was having turned out to be the 'camera connector cable'. I'd bought it off Amazon for about £8 or so... when I replaced it with an official Apple one (£25) it worked.
Here's what you need should you want to do this yourself:
AmazonBasics USB 2.0 A-Male to B-Male Cable with Lighted Ends Braided 1.8 m / 6 Feet
https://www.amazon.co.uk/gp/product/B00NH11YYK/ref=oh_aui_detailpage_o00_s01?ie=UTF8&psc=1
Lightning to USB Camera Adapter
http://www.apple.com/uk/shop/product/MD821ZM/A/lightning-to-usb-camera-adapter
