did you think of the power your going to need to push the 18"s?
how about the room to install them into your trunk....i think your going way over board..
i do have one ? thought....why 18's? there soo sloopy...cant beat the sound and tightness of a 10" sub,my personal car im running 2 fosgate punch hx2's and they have a hard time fitting into the car,makes getting the spare a rough rough adventure...

 

it would take more than 3 10" subs to equal the output of a single good 18" and far more power. You would also need a much larger enclosure for 3 10's to play as low. When comparing crappy off the shelf 18" subs then yes they can be sloppy, this isn't the case here though.

 

You might want to rethink the underdash baffles and leave it open. Sound absorbing materials will be a big help. The midbass frequencies will have a path length of around 4-10 feet. Any reflection off of hard surfaces within half of these distances will be potential problems.

A perfect example of this is the hole in response in the area of 150-250z that appears in systems with door mounted mids. The distance from the driver to the center console, dashis around 3 feet. So the wave comess off the cone hits the console and reflects back to the cone cancelling other waves of the same frequency. You can bring up the amplituded on an EQ all day long but the output will be virtually unchanged because as amplitude is increased so is the cancellation. Midbass placement is so critical because of the typical distances from the seats, console, dash etc. A driver the size of a 10" has such a wide range of dispersion that this effect will be amplified. I started leaving the underdash area open and acoustically damping the area to reduce the reflected waves and more importantly to make them longer so they didn't cause any of the typical problems. The size of the speaker will limit placement as well and will likely cause more of these issues to occur. If this is all done right a good sound stage will be acheived and most importantly you will have Stable imaging which I find is the real weakness in car audio.

As for the tuning, With the processing you have it is impossible to accurately set timing, phase and xover perfectly. There are just too many potential settings that all impact each other to do it. I bought an Audio Control IASYS to be able to do this and it gives accurate time measurements in Ft and Ms, shows the phase curve of the driver in car, and shows the useable frequency response in car with the Fc and recommends xover points. You wouldn't assemble a motor with an adjustble wrench and a pari of needle nose pliers and you should use the right equipment that allows you to properly set the processing power you have. Just my $.02

 

This is why i never got into SQ. Id rather crack windshields with dB.

dB--- Power + Size + Proper Port = FUN

 

I like to hear music, thats why I never wanted to get into SPL I would rather buy gas than windshields...