Thanks LT... but I really don't have 4 feet of space to spare at the back wall. I have total 24 feet from front to back. And I can't place the speakers 1/4 distance either cause that'll be 6 feet out from the front, which I only have a recess of 4 feet which i can use.
So, does this mean this whole concept is out the window?
Basically, from the drawing of my basement, you can see I have a 4 feet deep recess where i can place all the speakers. I'll have a 10 feet wide screen then, followed by first row at either 10 or 11 feet away, and then second row at 15-16 feet away, leaving exactly 4 feet of space behind. I can probably do a few inches of absorbsion at best. Someone once told me to put up a thick wood on the corners to trap the bass... for instance a solid wood about 1-2 inches thick. Basically, between the wood and the concrete wall, there's some space... and the bass might bounce around in there and loses strength...
Also, there's a complexity to my basement, ie, it's not a straight forward rectangular shape. There's a 'recess' of 4 feet, then it recesses in and out of the side walls a couple times as in the drawing. The back-wall is bigger than the front recess.
And plus, I already have an OS, and throw in 2-3 mid bass modules i want to build, and subs.. it's not looking like an easy solution. There won't be a clear 'space' behind the wall for the 1/4 spacing etc. I guess it's all down to experimentations