I've read through the thread, and have a couple of questions:

If you don't need amp modeling or effects, the easiest solution would seem to be an external headphone amp like the Vox Amplug or PocketRocket. In this case, iRB runs as the only task on IOS, and the iRB output is mixed with the instrument in the external box. Sound right?

If you want amp modeling etc, the iRig or Ampkit Link will send the instrument signal into the iPhone where it gets processed and mixed with the mp3 or iRB output. OK so far?

In the second case, either the iRig or Ampkit Link can be used with iRB playing, as long as you have the new version of the Ampkit app. Yes/No? Either of these hardware choice preferable?

Other hardware options are popping up:
* Line-6 Mobile In
* Sonoma WireWorks Guitar Jack 2
* Alesis iO Dock
These seem focused on recording, but anyone tried them for just practicing?

Thanks....Lyle