Hi and thanks for posting.
Many iOS app will work with the DGX660.
One useful app is the SoundController which emulates a pitch bend wheel and a modulation wheel but you can assign the controllers to other sound parameters such as cutoff and resonance, attack and release etc.
Of course you can use the DGX as a MIDI controller to control iOS synths such as Korg iM1 (paid) or Yamaha FM essential (free) and Yamaha Synth Book (free). You can then send the audio output of your iPad to the AuxIn port of the DGX to have the sound of the iOS synth coming through the speakers of the DGX.
Regards,
Vinciane