You know, there isn't a lot of material under 50Hz anyway, so having something strong above 50Hz is great and can sound thunderous. I only meant to point out that while the three π isn't a sub, it does generate deep bass. So for a sub to get underneath it and provide significant output takes something tuned pretty low.

A horn shifts to being a direct radiator below its passband, and since the rear chamber is a rather small sealed box, it has a hard time generating a lot of energy under that point. That's why I wanted to provide a suggestion. If you were running one π speakers or something smaller, a 50Hz horn would have made sense to me but in this case I would have preferred a subwoofer with lower cutoff, that's all.