I have tried my iPad‘s Garageband drumset as an external source.
There was actually no problem in processing the signal.
It goes directly to the delay/distortion unit of LYRA.
You just have to make sure to feed it into LYRA with an adequate volume level (in case of the iPad, more than 90%, so LYRA‘s voices would not drown it).
I sometimes run into the problem though, that LYRA‘s delay unit seems to become „stuck“ as in: no delay discernible anymore. Then I have to turn the FEEDBACK knob all the way clockwise until it kicks in again and starts to self-oscillate. Now turn it back to desired level.
Don‘t know why it sometimes gets „hung up“…