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 StoreAudiobus is the app that makes the rest of your setup better.
More about Unity and AB
Have been very pleased with Unity synth with yes all the IAPs, using AB with Beatmaker 2 (in both a 4s - for which it supposedly isn't optimized - and iPad 4) and Cubasis.
A really big, formidable synth with most of what one might seek elsewhere: sf2 instruments, and all types of synthesis including fm, wavetable, subtractive. There are 100s of great sounds from mono synth to 9' concert grande-
Some tricks are helpful. As with Alchemy, (in which, IAP-wise I've invested far more, and also happily) one starts Unity and selects a first patch before opening Beatmaker or Cubasis...changing patches once midi and audio are flowing involves stopping the DAW from the DAWs own GUI then browsing patches or tweaking in Unity GUI.
On the 4s I felt like it helped to keep some gigabytes free, for the big synth and DAW to flop around in.
Unity glitches ceased when I emptied out space and followed the AB order of operations.
The fact that I can design on the iPad and send these patches to the iPhone is sweet - something Crystal xt offers too. Alchemy meets Crystal and an sf2 player, meets physical modeling and and and...
Not to mention the keyboard does expand and contract in height and width per any need, and there are 4 xy pads and a Morphwizish style interface when keys aren't wanted...