PMC TLE1 has dual 6 1/2 inch drivers and a flat frequency response down to 22Hz. It's not a square box but a slim rectangular box that may fit your criteria of being small. It pumps out quality bass with the internal Bryston amps rated at 150W and totally kills my Dali SWA-12 with a 12" driver. Big drivers don't necessarily guarantee better quality bass or the ability to go lower down the frequency ladder.
