See VocaLive (with iRig MIC for vocal input) and AmpliTube (with iRig HD as input for guitar and bass) with great apps like Audiobus (of course) and GarageBand in this great video. See how easy it is to connect VocaLive with other apps that are compatible with Audiobus.
Audiobus (from Audiobus Pty Ltd) allows the processing and output of audio apps to be used in real time with other apps, thus "connecting" them much like audio cables connect pieces of gear. Singers can now use VocaLive to add stunning vocal effects to their recordings in other apps like AmpliTube or GarageBand, or add instrumentation from other apps to their recordings in VocaLive.
VocaLive for iPhone, iPad and iPod touch and AmpliTube for iPhone, iPad and iPod touch are now Audiobus compatible. Audiobus is a groundbreaking new app (developed by A Tasty Pixel in cooperation with Audanika) that allows users to connect the audio of multiple apps, opening up countless new sonic possibilities. For example Audiobus lets users send audio out of one app, filter it through the next one in the chain, and receive it in the third. Vocals can be run through a filter or multifx; or a synth could be looped by a looper and its output sent to a recorder; or an electric piano could be run through a guitar multi-FX app and then into a sampler — all in real time.