OP, if you can find frequency response charts they will help you. The cubes drop off around 200-250Hz. That would mean you need to compensate with the sub. Unfortunately the receiver only maxes out at 150Hz. Other problem being, the higher your crossover point the muddier the sound becomes.
My reccomendation: set speakers to small and crossover at 150Hz. See if it's too muddy and lower your crossover point if necessary. But keep in mind, there's going to be a frequency gap between 150-250Hz which just worsens as you lower your crossover point.