Played around with Hornresp for a while, then looked on eBay. Then played on Hornresp some more.
Dropped a (slightly used) Eminence Kilomax 15 in there. Gives a similar curve to the RCF driver before (dB or so here and there), so I called it close enough. I bought a pair for less than the price of a new one.
Threw one in there and the results are pretty impressive. The living room is weird for bass - more nulls than peaks - but there's solid output to the high 30s. I've got one of the drivers on break-in mode as I'm not convinced they're fully broken in yet. Bass I Love You low-passed (you read that right) at 20Hz gets the cone moving without disturbing anyone.
I must say that the Kilomax is a beast. The lengths they've gone to in order to get rid of heat is quite impressive. The holes in the backplate go directly to the 4" (!) voice coil, and the heatsink out front shows promise.
Something to note, however, about these drivers: the suspension really puts the brakes on when you try to exceed Xmax. Harmonic distortion is immediately apparent. The audience probably wouldn't notice in a PA application, but for (very very loud, or open baffle) critical listening, its worth bearing in mind.