[mhauru]

Sorry, I could have been clearer in explaining how to actually use the thing. So the keyboard at the bottom is in the regular 12-EDO tuning that we are all used to and cubase does as well. It's the red dots that are the just intonation tones.

To hear the difference, try playing a just intonation C major chord, by clicking the red dots labeled 1/1, 5/4, and 3/2 (hold Shift for sustain to have them ring at the same time), and then the same chord in 12-EDO by clicking the piano keys directly below the tones you just played, so C, E, and G. The difference isn't massive, but you can probably hear it. If you want to make it more obvious, click the gear icon on top right and choose the sawtooth waveform, and try the same thing again.

You can also try playing a 12-EDO E from the piano keys and a 5/4 from the just intonation dots. These are the same tone in the two different tuning systems, and you can hear the "beat" between them, the slow oscillation that comes from them not being quite at the same frequency.