I am not against using 18s, the general theory all along is more voice coils make it louder (lots of 10s, like Alama Gates PPI Iasca Bronco), or using one or more 15 or 18 and this allowing the bass to sound lower because they are larger cones. It is just that you will need to remove a seat or two to house an 18 inch or two to get the right air space in a normal vehicle. Also, to control the driver you need mass amount of clean power and plenty of juice from your alternator, isolator and a 100Farad cap with multiple Optimas. I love deep, tight bass, our theater rocks. It is just that 18s are more often found to sound best at concerts and in gentlemen clubs and other dance clubs. Plus, the car, the worst acoustic envornment in this world, and an 18 is ear-damaging, but some like it that loud.