What room treatment product have transformed your room or audio system?

interesting topic. I have my wall behind my speakers treated and the  room has carpet and furniture. it also has a 1 ft. soffit from the ceiling that divides the room in half that faces the speakers.  should I be treating the flat surface of the soffit also?

 here's what I did for a start on the 1 ft. soffit that faces the speakers. I bought some styrofoam/spongey like gutter guards from home depot and layered 3 of them to the top of soffit all the way across, which is 12 ft.  It has improved the SQ. I can hear individual instruments more clearly with more sustain. I'm going to cover the rest of the soffit with these and see what may or may not come about. my guess is the flat surface of the soffit was reflecting frequencies back at the speakers and canceling out those certain frequencies?