What you're going to need is a huge displacement, not electrical power per se, i.e. multiple infra sub-woofers, which will require increasing system efficiency enough to get to 10 Hz unless you know of any super low Fs 18" (~10*sqrt(2) = ~14 Hz at maximum for a sealed, vented alignment, compression horns up to ~ an octave higher) with apparently currently unavailable high Xmax as I couldn't find any < 25 Hz Fs and/or > ~23 mm Xmax in a quick search, so at this point using a number of relatively small subs per Dr. Geddes' research to optimize their position, sum to whatever peak SPL desired seems the only viable, best overall option.
Was Looking at 21" drivers but am bit worried about the Width required. 23" which is the Minimum required without modifying the driver by grinding two sides of mounting plate is 1" thicker than the space available i.e. without obstructing path of the door-way. Will have to consider 21" driver with or without the Mod. I think 1" is not much and won't be noticeable as the door opens the other-way.