Regarding your sub box, pull the rear seats down, and feed them into the trunk that way. It'll be a huge pain to take them in and out, but that's one way to get over the small trunk opening.
For what it's worth, I was able to fit 7 people in my 94 Mustang, for a 3 hour ride.. 3 in the front, 3 in the back, and one small girl in the trunk
You just have to get creative.
