Hi Matt -
It sounds like the amp output on the iRig is functioning as a splitter to take the incoming [clean] guitar signal to the amp while simultaneously sending it to the DAW for recording. Apparently there are two ways to do what you want..
- According to the iRig manual you can select iRig as the output device [in GB's audio Preferences], and then take the signal from the headphone output [not the amp output] and send that to the amp [with a suitable adaptor] -- see the iRig manual p.7.
- The manual also suggests that you can send a processed signal to the amp output by setting the Thru-Fx switch to Fx [iRig manual p.8-9], but I'm not clear if that's the full GB output or just the processed guitar itself [you'll have to try it]..
Btw since GB is the DAW I've moved this thread to the GarageBand subforum..