Its not so much that you need a gutsy amp to drive a 12" speaker; what you are dealing with here is the simple fact that JBL chose to make this speaker hard to drive. Its nominally 4 ohms to start with, and has a difficult phase angle in the mid bass region.
Due to the 4 ohm impedance the efficiency translates to about 88dB. This means that in an average room the amp will need to make about 200 watts (into 4 ohms) to be able to play at lifelike levels.
Now you may have an issue with having one speaker wired out of phase with the other, so I would check your connections and make sure both speakers are working together rather than against each other- if the latter is true, it will be a bit bass dry!
Due to the 4 ohm impedance the efficiency translates to about 88dB. This means that in an average room the amp will need to make about 200 watts (into 4 ohms) to be able to play at lifelike levels.
Now you may have an issue with having one speaker wired out of phase with the other, so I would check your connections and make sure both speakers are working together rather than against each other- if the latter is true, it will be a bit bass dry!