Diffusion versus absorption behind speakers?

I had a full "Live end - dead end " set up at one time using Sonnex. I liked it at the time. My present room is 35'x13'x7' and had serious problems. I would also start with bass traps in the corner and go from there. I eventually added 7 more, mostly on the back wall as well as some wool area rugs. the room is still quite live but I can now hear things I never heard before. The amount of detail lost to cancelation due to reflections will astound you. I got mine from Ready Acoustics, who were very helpful, they also will send you a suggested set up if you provide them a picture. They are reasonable and my wife likes the look of their products. No connection, just satisfied customer.

Depends on room, radation of loudspeaker, no right or wrong just whats right for your room system etc.

Why spend extra money? I placed tall (7 feet 6 inches tall, 24 inches wide, 10 inches deep) bookshelves in the far corner on both sides, filled with stuff (various books, DVDs cd). Behind the speakers, in the middle is a low leather chair Floor length curtains are opened on the back wall.
The room sounds boomy without anything in the corners. With the shelving it sounds more natural.
I am amazed that folks just do not use the normal home furnishings to 'fix' room problems. Does spending a few thousand on 'special' crap makes them feel better about what the stuff can or cannot do?

Thanks everyone for your responses. I am not a novice on this subject. I know treating corners of the room and back wall behind the listening seat with bass traps has beneficial effects to tame room modes. Also treating first reflection points help in imaging. But I have a specific question regarding the front wall. Some use absorption that helps with better pinpoint imaging. Diffusion, I heard, makes the sound stage bigger but images get larger also. I probably will end up with a box speaker and a panel speaker. So I'm trying to to see which front wall acoustic treatment will help both types of speakers. I get the feeling absorption behind dipolar planar speakers may not be a good idea.

I am more inclined to go with diffusion regardless of the type of speaker involved. As I stated before and Elizabeth has noted too, use bookcases, furnishings, wall hangings, etc, before looking at acoustic treatments. If one of the walls is a big glass window or door, that should be a priority (use blinds or curtains, etc.). If possible, avoid having a very big and flat reflective surface (like a coffee table) between the listening position and the speaker.

Another "free" fix is to listen in the near field by sitting fairly close to the speaker so that direct sound of the speaker dominates over the sound contributed by the room. My preference on that is to move the speaker further into the room. I almost always prefer the sound of speakers when they sit well away from the wall behind the speaker.

I consider extensive experimentation with placement of the speaker and/or listening chair to be the first, and most important step to getting good sound. Almost all difficult "room" problems can be substantially ameliorated with proper placement of the speaker and/or the listening chair. It is surprising how much even a small movement of the speaker, or change in toe-in or rake angle (how much the speaker is angled backwards) changes the sound. If random trial and error is way too exhausting and frustrating, you can try some of the more systematic approaches. A discussion of these approaches is a BIG subject area (google the "Sumiko" or the "Wilson" method of speaker placement).

If all of the non-acoustic treatment approaches fail, start first with tube traps or corner traps in the corners of one side of the room. Unless you build your own bass traps, these can be somewhat expensive. I am personally not inclined to go beyond bass trapping because the costs can get really high and the results are often inconsistent. Plenty of junk on the walls to diffuse sound usually works as well or better than specialized treatment.