Quick question about a box in the bed of a truck. Will 3 subs, outside two with a slight angle, middle one facing straight..take up a decent amount less room than the 4 subs in a pacman. Old lady wants me to save a little room for groceries, maybe a bag or two if we need it. They are 15" BTL's fully loaded.

Basically, you need a certain amount of airspace per sub. I'm not familiar with FI, but I know you absolutely don't want any LESS than 2.5 per sub. More will be more efficient with more space, but you should just ask Shawn Flemming. Didn't you get those from him?