Open baffle has rear output which allows the speaker to take advantage of the ear/brain system's abilities to find the source of the sound- in effect, the reflected sound improves the soundstage.
They can make bass too if properly set up. However it does seem that you get the best results if the output impedance of the amplifier is higher rather than lower! A tube amp that does not use negative feedback is recommended- this will put the output impedance in the right range and it will be easier for the speaker to make bass.