Most rooms don’t need acoustical treatment.

Hello,
Bottom line it does not matter. We all spent serious money on our systems yet The chances of even 1% being the same is rare. All that matters is our own ears and the preference of sound that we prefer or are reaching towards. I have heard inexpensive systems that sound great and really expensive systems that do not. I am an advocate of some room treatments. That is because it sounds great to me and several other people. 

I think it matters a lot. In 2017,  I reluctantly moved one piece of furniture and it made a big difference. I would say 100% difference. The system sounded twice as good. Moving my LP rack to the first reflection point on the left speaker was like finding the autofocus on a camera lens. And I simply rearranged the stuff that was already there.

That change alone made me chase optimum room configuration for 4 months.  I didn't use specialized products. This is how powerful a room can be.

I just never thought it could make that much difference all these 30 or so years...I would move the speakers around a bit and adjust a few things and if it sounded good, I thought that it was set. Didn't think to try a little harder. 

I agree with erik.  If this silliness is not kenjit then it must be his brother.  It's just dumb.

Ok, now I learn more.

absorption done by couch rug and curtains, yeah.

but seems that diffusers have great value,  ie.  They scatter and add clarity to sound waves, and make room seem bigger.  It’s mystical, no one knows why but this seems good.  A panel will do better than a bookcase, so I buy diffuser.

@jumia -- I think you misunderstand the general direction of this thread.   

The subject of room acoustics is well understood by those who have studied it extensively. However, it is not an issue where an amateur can do a five minute Google search and have any assurance that they've come up with the correct answer for their particular situation.   

There are lots of things like this in life.  You want your airline pilot, doctor, structural engineer, and so on, to have specialized knowledge and training for their job.  It is certainly easy enough to look up info about all of those subjects on the internet, but that hardly means that the person who does that is ready to fly a plane, operate on a person, calculate load bearing specifications, or do the many other things that people spend years training for.  

Acoustics, and all those other things, still may be mysterious to you, but that doesn't mean that others out there don't have a good handle on things.  

But, that's the nice thing about this hobby. You can experiment to your heart's content with no concerns about the plane crashing, the patient dying or the building collapsing. Who know, you may even end up improving the way your stereo sounds for you!