It could be too late..! But I had the exact same problem and here is how I managed to solve it:
IRig HD is compatible with the range of IK AmpliTube apps (the #1 app for guitar players) including AmpliTube Fender®, AmpliTube Slash and AmpliTube Jimi Hendrix™, as well as any other real-time processing apps, such as Apple GarageBand, which support digital audio. Use the iRig HD anywhere you play guitar – on the go, in the studio or on.
Voila!... It should now have been perfectly recognised by the computer via the headphone's mic and it will now work even if you unplug the headphones from it.
Hope this solves your problem too. Good luck!