I am using the Visual Metronome app on the iPhone which is great but it does not quite have enough volume. I could use wired or even wireless earbuds/phones but they restrict being able to hear the piano sound. Instead, I am sending the audio to a most awesome personal wireless speaker, the Jawbone Jambox, sitting on top of the piano to amplify the output so as I can hear it more clearly.
I also send the audio to the Jambox to listen to the hundreds of scores, I have on the iPad whilst using ForScore to view them. It does so very well, is quite functional to use to play from also and has an auto page turn facility.