If you feel like some experiments...Try these iterations and see how each one sounds. Make notes.
1a) Move the 18" sub (I think I see it along the left rear wall). Put it right between the two seats, as if an end table, facing forward. Feed it from one of the sub outputs. Feed the 12"subs from the other sub output. Run the XT32 setup.
1b) Turn off the 12” subs. Run the XT32 setup
2a) Move the 12" subs, placing one behind each seat, facing forward. Connect them to the same sub output. 18" sub turned on. Run the XT32 setup.
2b) Turn off the 18" sub. Run the XT32 setup
3a) Move the 12" subs to the side walls, flanking the seats, aiming towards the seats. Run the XT32 setup. 18" sub turned on. Run the XT32 setup.
3b) Turn off the 18" sub. Run the XT32 setup
Deadwood Atmos theater
AV7702 Atmos 7.4.4, SSP-800 PLIIx 7.4
Aerial Acoustics 7B/CC3B fronts, B&W CWM8180 surrounds, Tannoy Di6 DC heights, Hsu ULS-15 subs