I am in the process of building my car and I am wanting to do something different. I started out with 3 solox 12's but traded them for 2 solox 18's. I have 4 crunch Gp3000d's to use but I only have 18 cubes total to use. So i thought of using only one solox 18 with 2 GP's in a 4th or 6th order bandpass enclosure. This is for daily and maybe a once in a while bass race/street beat run.

Would it be better to run 2 18's ported or one in a bandpass?

If a bandpass how big sealed and ported.

have you thought about a t line?

Vent them and be done with it. No room for a proper T-line. No need for a bandpass.

Why did you buy a solo-x for daily use?