Most rooms don’t need acoustical treatment.

@asvjerry 

It's 'sand them all'. 

Back in design school, I was in line in the shop to return some tools. A friend of mine walks up and asks the guy in front of me if he should sand just one of the pieces of wood for a project.
The guy responds by saying 'just sand them all'.
Then they both pause as a light flickers in their nerdy eyes before they burst out in laughter.

My name is Sandamal. It's a perfect phonetical pronunciation of my name. And once again, as it has so many times, my name has betrayed me.

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.