Couple of projects of mine - not sure if the photos will load OK.
First is a pair of 25Hz horn subs I used to have - used 18inch p.audio drivers, placed facing one another along a wall to form part of the horn and achieve adequate mouth area for the 25Hz cutoff.
Efficiency was well over 100dB/w/m.
Second photo is one of my digital crossovers I built, 4 way AD1940 based audio DSP with an AD1939 CODEC as A/D and D/A. Works a treat, and makes setting speakers up a snip, as can program in delays, HPF / LPF, parametric etc.
Also included my subwoofer couch - I built this as a challenge from a colleague of mine. The carcass of the couch forms an 800 litre enclosure, the ports exit at the back of the headrest three of them, and I used four fifteen inch drivers. It is tuned to 19Hz, and achieves a -3dB point below 20Hz.