Hi Tim !
Thanks for the praise

.
For your problem, this is most likely an iRig problem. It is known for its bleeding from its output cable to its input cable. The AmpKit tries to solve that by adding an amplifier at the end of the output cable, so you can lower the volume and thus the bleeding. If you re-do your test with an iphone output volume set to 0, you should not have any bleeding. But of course, this is not a viable solution ^^.
To get rid of the problem, you probably would have to upgrade to a better quality - and more expensive - audio input device.
Cheers !
Raphaël