A priori: relating to or denoting reasoning or knowledge which proceeds from theoretical deduction rather than observation or experience. It's the name of Bulgaria's open baffle speaker based on a concept by Raal Ribbon's Aleksandar Radisavljevic. What's unique about it? The Ripol cell implementation for the dual 15" woofers. It reduces the usual housing we still see in Voxativ's Pi Bass or the MusicBass subwoofer from ModalAkustik to the barest possible minimum. We're now down to half a hub cap over the drivers' side sections fronting the baffle. It's the most elegant way of controlling front and rear Ripol radiation I've yet seen. That makes it worthy of another mention; and to perhaps prompt others to investigate it. It's amazingly effective and visually limits how much space two 15-inchers take up.
If you want truly quick bass without the typical energy storage of box bass or its obvious overhang from in-room omni propagation, such close-proximity slot loading lowers its drivers' resonant frequency and maximizes out-of-phase cancellation beyond standard dipole bass. One thus needn't add a standalone Ripol subwoofer like we do with the sound|kaos DSUB15. One needn't worry about another amp and outboard xover. One can get such bass built into a passive speaker. The Dragon Egg from Cygnus Audio of Germany offers it in active guise. And yes, it makes for very obscure company either way. All the more reason to see wider adoption and keep stirring this kettle. After all, as a big proponent of dipole bass, Siegfried Linkwitz is no longer with us.
The two lower photos are from Dawid Grzyb's review. Revisit it for his sonic commentary to fully appreciate today's mini rerun of it.