I have some rock wool (8lb) four inches thick on the diagonal of the corners behind the speakers. I could not believe the difference it made. I highly reccomend it, especially if you have squarish room dimensions. I haven't personally tried all four corners, but if I could I would definitely do that as well. The other thing to think about is the first reflection from the ceiling. If you have speakers with good dispersion in the tweeter, you should put some damping in high frequencies on the ceiling, in my opinion. Ambience is good, first reflections are bad, again in my opinion.
bass traps..which wall first
when using bass traps (any kind for this question/answer) which wall should be the priority, the rear corners behind the speaker, to the corners behind the listener? Can 2 different ones be used "safely" for example can you use 4' tall traps behind speakers, and 2' behind listener and vice versa?