We have a very large room with high ceilings. The ability to connect a second Pro Sub would be an awesome feature.
I would buy at LEAST one more Pro sub, and probably TWO of the smaller ones.
The frequency of the smaller subwoofers have to be different because the drivers are smaller, thereby filling a gap in the total sound picture.
I would think that the ability to pair multiple subwoofers would add several million dollars to the Roku coffers.
My first purchase would be a second 10", and one of the smaller ones, putting BOTH next to my recliner.