I can concur about the importance on treating the ceiling. I have just put these on first-reflection points on the ceiling in my listening room: Flat Panel PET - Vicoustic

They really made a positive difference. Much more than the panels on my walls (some more Vicoustic Flat Panels and four GIK Polyfusors), which frankly disappointed. I already had an inkling that the ceiling panels would be good bc I had tried to temporarily put polystyrene diffusors on first-reflection points on the floor, and that made a big positive difference.

I can highly recommend the Vicoustic Flat Panels for both wall and ceiling mounting. The come in different colors, and can be attached with velcro tape that's included in the box. Just don't put them right next to each other, as they are difficult to align flush.

I'm now looking for some kind of thicker rug(s) that doesn't look awful to put on the first-reflections points on the floor.