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.
Use Layers to isolate bass/treble clefs
When writing some music for an instrument I like to be able to hear the bass line or upper melody in isolation. You can achieve this by using two tracks for bass + melody and mute accordingly, but for an instrument like the piano then it makes composing easier for me if the MIDI is together on one track.
Using the 4Pockets Layers app you can easily define a couple of layers which cover the required 'split' and then double tap the one you want to mute, for example (MultiTrackStudio score view with Layers MIDI FX plugin visible):
Here, I have defined two layers A1 and B1. The 'split' between bass and treble clefs in MTS has been configured to "C5". If I double-tap B1 then the all of the bass notes in the first bar are muted. If I double-tap A1 then all treble notes in the second bar are silenced. This way I can focus on bass or upper melody in isolation.
I have not tried it but you could do something similar I suppose to isolate the different parts of a multi-instrument drum track.