GIK Acoustics art panels and Impression series panels in as many places as you can stand.
GIK panels on the ceiling between listener and speakers.
If I understand your room, it's 18'x45' with 18' high ceilings? If so, consider bass traps at the intersection of the floor and end wall, based on the first few room modes in the AM Coustics calculator
https://amcoustics.com/tools/amroc?l=18&w=45&h=18&ft=true&r60=0.6