OK. I've finally figured it out.
If you have your soundfont playing and you look in the mixer, you'll see the DLS Music Meters alongside the faders moving, along with whatever Built in Audio Track is outputing the signal as well.
Let's say for example if you placed your reverb on aux 2 and built-in audio 3 was registering audio signal, you would turn up the fader on aux 2's built in audio 3 and wham you have reverb on your soundfont!
Cool. Nice implementation.
Nice bright new silvery icon too for Metro's app.
scoot.