The STF-1 will go down to 29hz with a +/- 3dB window. Hsu lists it as 32hz in a +/- 2dB window, which is more prescise than anyone else lists specs. TheF12 can only be said to hit ~28hz within a +/- 3dB window when measured accurately. It would just have more output than the other 2.
The differences between the M8 and STF-1 are slight, as they were both designed the same way. Outlaw contracted Dr. Hsu for consultation on the design of the M8. The STF-1 will have slightly more amplifier power and slightly more weight to it. Otherwise, they are pretty close. Both are good for their price class.