I found similar with strategically placed panels. Similar to the ones you have {built mine with wood frame stuffed tightly with thickest highest rated insulation Depot had}. A little extra work inside the panels to get insulation as far in or off wall as possible. Behind seating and first reflections made my stage expand immediately.
Now use a 6 foot book shelf all Blueray, books etc. on one side. Helps a lot with such trickery.