Audiobus: Use your music apps together.
What is Audiobus? — Audiobus is an award-winning music app for iPhone and iPad which lets you use your other music apps together. Chain effects on your favourite synth, run the output of apps or Audio Units into an app like GarageBand or Loopy, or select a different audio interface output for each app. Route MIDI between apps — drive a synth from a MIDI sequencer, or add an arpeggiator to your MIDI keyboard — or sync with your external MIDI gear. And control your entire setup from a MIDI controller.Download on the App Store
Audiobus is the app that makes the rest of your setup better.
PSA: Don't update to iOS 12.4 if you use apps in AB output slot or IAA apps that use the mic.
It may be too late for some of you, but I've just discovered a late-breaking issue with iOS 12.4 which seems to affect at least some output apps and their ability to receive audio through Audiobus.
Still investigating, but for those of you lucky enough to still be on pre-12.4, I urge you to hold off updating for now. Will update this thread with details as they emerge.
Update: I've applied an emergency stop-gap, which forces manual "Tap to launch" launching of all apps that can be put in the audio output slot in Audiobus. This seems to resolve the issue for now, although it's a little inconvenient.
The stop-gap has been applied immediately, as it's a modification of the Audiobus database, so it will work right away.
Update: The issue discussed in this thread is fixed in iOS 13.1, and in iOS 12.4.2.