Start with bass traps to get the low end under control first. This should be the first step in treating a room. Measure the room as is and then as you add treatments so you aren’t just guessing. You really can never have too much bass trapping. Go as big as you can everywhere you can.
Wool Blankets as Room treatment
Just noticed Pendelton has some really nice wool blankets that can be hung on the wall. They even sell the fixturing. I would assume the wool blanked would have some beneficial properties for a blank wall?