2 AQ SDC2.5 15's or 1 AQ HDC3 18
 

Both would get 8 cubes at 36hz with 1500rms. Copper or aluminum for the HDC.

why not 2 15" hdc3s?

^ I'm guessing due to rms power available vs. rms power handling of sub choices.

My question would be, what are your goals? Strictly spl wise, I'd say the 1 18" HDC aluminum. If your vehicle is going to be used as a regular driver as well, I'd go with the dual 15"s.

woohoo my vote is winning!!! lmao

two 15's

Two 15s if you got space for the box.

Why are you (those who voted for) recommending the aluminum version over the copper? Wouldn't the copper be better? (I assume the copper vs aluminum is about the voice coils?)

It says on the site that alum's are better for SPL, and the coppers are cheaper. Get the 2 15's, more cone area with a +3dB starting sensitivity. Read the reviews, people are saying they get really low even when tuned high.

I didn't look at the site or price, but the aluminum coils should disperse heat better.