You're right that a bass-reflex cabinet tuned too low doesn't do much. If the Helmholtz frequency is tuned low, it forms an EBS alignment. But if it is tuned lower than that, it begins to become ineffective. It's tuning is sort of "out of bounds" and it begins to more like a leak than a tuned port.But this cabinet is designed to be tuned to 25Hz. The response curve is similar to an EBS alignment. It begins to rolloff slightly around 50Hz, but has a lot of energy all the way down to 20Hz.