24' x 30' with 9' ceilings, that's almost 6,500 cubic feet of space. That's a big room. You'll be gald you got 2 "monster" subs. You can try them under the bench seats in the back, but be sure you cross them low enough that they're not localizable behind you, (80 Hz or lower). What are your mains, as that will help determine how low to cross them over.
The other place you could try is along the lower wall in front of the double door, (closer to the front of the room). Placing them together, (co-locating them), will give you a little more output, (~3dB). Have you run the sub cables yet?
Also, is your screen acoustically transparent? Since your front speakers appear to be behind the screen, the screen should be acoustically transparent.
Good luck,
Craig