MIDI is a digital signal. Anything you're going to hear out of a PA is going to be from an analog signal. MIDI does not produce sound, but only transfers a digital notation to a host, which will apply it to the designated virtual instrument, therefore producing an analog output that produces sound. This output would then be sent to the PA, using 1/4 instrument cables in either stereo (L and R mono) or single mono.
The only way I know of that can take a MIDI signal and apply it to an instrument is by computer. You will then need an interface with outputs that can be sent to the PA.
Basically, it'd work like this:
MIDI Controller (Your MIDI "instrument") output > MIDI input of interface > firewire into laptop > Reason or whatever MIDI program/plugin you use > main outs of interface L and R to PA, where channels should be hard panned L and R