Its not practical to get every speaker in a two or three channel setup to go that last octave. I use a 15" for its high efficiency and directivity not for its LF capability, although that is a plus. But a cabinet to 25-30 Hz for a large high efficiency woofer is not practical.

But it is practical to design a sub using a smaller woofer in a bandpass design that goes down to 25 Hz. It has a 25-50 Hz capability, matches up ideally to the closed box 50 Hz Summas, and has enough output over this very small bandwidth to keep up with the high efficiency woofers in the main channels. It probably does have a lower MAX_SPL but it will still do 110 dB SPL in a small room at 35 Hz. Total volume of this approach is far below that of making each source go down to 25 Hz.