Beginner MIDI keyboard question?

I'm interested in expanding the sound samples of my keyboard (Privia PX-3) for live performance via amp (Roland KC-550 connected to the Privia via instrument cables). I have heard that this can be achieved by using my keyboard as a MIDI controller. Toward that end, I'm considering using something like SampleTank and iRig MIDI as a "sound module" for the keyboard.

The question is, I can see how this would play the additional sample sounds directly on the iPad, but how would I then play these through the amplifier? I'm unaware of a setting on the Privia that allows for it to play anything but its own preset voices, and I'm assuming something like that would be needed to get the sound properly to the amplifier.

Is that a correct assumption, i.e. that the Privia would need to receive the sampled sound input from the iPad "module" via MIDI to play on the keyboard (if so, how??). Or, alternately, would there be a way to output the sampled sound from the iPad to the amp (without having it sound really bad coming from the iPad's headphone jack assuming that's how it would have to go)?

I've read where this supposedly can be done but can't figure out how it is from what I've seen--thanks for any thoughts on a direct solution to this or any workarounds as well.

I know the answer to this is somewhere out there probably in a pretty obvious location, but I cant seem to find it. How do I/where do I go to redeem my Online Enhanced Content codes? Thanks.