They are very nice looking and remind me of my old RH Labs subs from back in the late 70's. There is a problem and that is down firing drivers are inherently non linear. Because of the weight of the moving part (Cone, voice coil, spider, etc.) the cone tends to go down farther then it goes up creating harmonic distortion. This does not necessarily sound bad, but it is distortion. Most subs use vertical drivers now for this reason.
300 watts will not be adequate if digital room control is used. The computer is going to boost the deficient frequencies as much as 10 dB. That requires 7-8 times the power. The way around this is to use multiple subwoofers the more the merrier. I use four enclosures with eight 12" drivers in a room 16 X 30 feet.
The best designs now use opposing drivers in phase with each other. This is call balanced force design. The Newtonian forces of the drivers cancel out minimizing resonance. KEF, Martin Logan and Magico make balanced force subs.