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.
I was listening to the Gaz Williams interview on Mylar Melodies' Why We Bleep podcast and he made reference to the Elektron Analog Heat being his favourite thing that generates no sounds of its own but makes anything that passes through it sound better.
In hardware I'd say that was my ancient Roland PA-120 desk, but what are you favourite iOS apps for doing that?
I tend to love tape saturation so something like the three Tape Cassette plugins from Caelum are great or perhaps something similar - DAW Cassette from Klevgrand or TB Reelbus.
I guess an additional question is: what's your favourite app to add interest to a dull sound? It could be the same as the answer to the first question, but I'd probably go for a filter like the RRS Polivoks or even the spring reverb from iVCS3.