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.
Amp Switch - MIDI footswitch app for Marshall DSL40 and DSL100 amps
Hey everyone, I just released a new app for controlling the Marshall DSL40CR and DSL100HR amps with MIDI. You can connect your iOS device to your amp with a MIDI to USB cable and switch your amp’s channel, fx loop and master volume.
The cool part is it has the Apple Watch app which lets you control your amp from your watch!
I know there are lots of people here playing guitar. I’m not sure that anyone using DSL amps but wanted to share the news with my ab forum fam 
The app is free and open sourced. Here are the links:
App: https://apps.apple.com/us/app/amp-switch-midi-footswitch/id1630403022
Source: https://github.com/cemolcay/DSLSwitch