An iOS device (with headphone jack) can be converted into an effects unit by the creation of an Effects Insert adaptor cable. This cable is a combined Passive Sum, and Passive Attenuator. The summing ensures that both left and right channel out of the iOS device is mixed and does not cause clipping, and the attenuation ensures the line level from your synth or drum machine does not overdrive the mic level input.

The important function of this cable is to provide the correct voltage, gain, and impedance for the inputs and outputs of devices.

There are a tonne of audio mangling apps available for iOS, and this will help make them part of the signal chain. Personally, I use this hooked up to the I/O of an Elektron MachineDrum to provide some extra sonic variety in my patterns.