Panels and placement.
Placement isn't critical, but cover your first reflection points first. This is where music from your speakers will bounce off flat any room surface, wall or ceiling and be reflected to your listening position............think of it like playing pool, it's a bank shot with sound instead of a pool ball.............Easy to locate these. Have someone sit in your spot while you move a mirror around the walls and ceiling.........I did it by myself, but it's faster and easier with a helper....Any place that you can see either speaker in the mirror from the listening position is a first reflection point and should be treated. After that treat as many corners as you practically can........each room is different. You'll get the best effect by straddling the corners, wall-wall as well as wall-ceiling with the panels. Panels that will straddle corners should be left open in the back to allow the sound to pass through, bounce off the surface behind it and come back. This effectively acts as a deeper panel and you'll get better low frequency absorption from each box that way.
I wasn't certain what or how much to expect so I built and installed a few at a time, then played music and listened..........It just kept getting better, so I kept building panels..............Keep in mind that there is a point where you can have too much of a good thing. The music will begin to sound worse or dead instead of better.........That's largely a personal preference, so take your time and when it sounds right to you, or you think you maybe went too far, just stop.
My room is 14x23x7.5 feet and I installed a total of 22 panels around the room and ceiling. Thicker panels soak up lower frequencies, panels straddling corners tend to behave acoustically like a thicker panel. Different types of rock wool or fiber glass will absorb sound slightly differently, but probably not enough to matter in the grand scheme of things. I bought Roxul, 3" rock wool from Lowes because it was readily available and inexpensive, but as mentioned there are other options. Easy to cut, I just used a sharp, serrated butcher knife and it cut just fine.
My system is probably around the $30k MSRP point and the money I spent treating the room probably made a bigger difference in bass, sound stage, imaging, detail and overall quality than any single piece of gear I've ever bought.............for 500 bucks............Music that I had listened to many times and thought I knew, as well as my system, sounded so different with may recordings that it was like I hadn't heard it before.........you can't go wrong my friend and acoustic panels don't need to be "burned in"....although I wouldn't be surprised if some one here disputes that!! LOL........try it, you'll like it