You may want to look into the Tyler Acoustics Pro Dynamics line. These are not single driver, but the Eminence drivers used in them are super-high efficiency (about 101-102dbs). The mid-line PD-20s reach about 42hz on the low end (20K on top) and go for about $2800 new. You might be able to buy their larger bretheren used to get lower bass extension if needed, but the line is still relativel new so there are not many (or possibly any) available used.
I also recommend the Audio Notes already mentioned. Very good bass extension, especially if you are placing them at or near a corner...
---Dave
I also recommend the Audio Notes already mentioned. Very good bass extension, especially if you are placing them at or near a corner...
---Dave