- Apogee Jam or iRig guitar interface adapter for iPad , iPhone, iPod touch, Android and Mac
- Garageband (or other app for effects and amp like Amplitube)
- iReal Pro
- Choose “Guitar Amp” and turn on the monitor
- Adjust gain
Then select 'Copy' (or Audio copy in older iOS versions) from the share sheet options and paste it into GarageBand.
In iReal Pro, go to More/Background Audio and make sure that it's set to ON.
For an audio app to run background audio, it would have a setting for background audio OFF/ON. To run another audio app in the foreground at the same time, this app must allow the background audio through (many apps do not do this so you have to check.)