Josh,
You'll need to be more specific. EAW LA400 vs. which JBL dual 18's? and what is the intended application? I am sure you realize you are comparing a horn loaded cabinet to a bass reflex design.
That aside, some general observations can be made. For example the LA400 (single cab) will go down to approx. 45Hz and the JBL SRX4719X, will go down to approx. 30Hz (if that is in fact one of the JBL models you are considering). Single box to single box, the JBL will go lower. Couple two or more LA400's together and they'll go lower than 45Hz...probably somewhere in the 35-40Hz area if I were to guess (properly powered of course)
Output wise, the LA400 will net you approx. 2-3 db greater SPL and greater throw. The LA400 will also sound notably more responsive and tighter. The LA400 is approx. 125 pounds and the SRX4719 just shy of 190 pounds and will occupy more space in your truck or trailer. The LA400 is significantly more efficient as well.
Clearly you can see they are two different beasts and if you heard them side-by-side you would notice their differences. However since you are looking to replace existing horn-loaded subs, you should definitely have a listen to the dual 18's - they WILL sound differnt than you current horns as well as the LA400's.
I am a huge fan of EAW products including their bass horn systems such as the DCS2, LA400, BH822 and the awesome KF940. The KF940 is, in my opinion, the best bass horn available.
Again the intended application is a must-know to help you with more specific feedback.
Scott