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.
Arturia Oberheim iSEM is quite good
I just want to mention if you’re into analog sounding synths, the Arturia Oberheim iSEM is quite authentic. The more I mess around with it, the more I’m impressed.
it’s just too bad Arturia doesn’t make more for the IOS music world. It’s also AuV3.
Depeche Mode classsic synth.