okay, thx for your input, nms! So I am going to plan a multi layered limp membrane absorber for the back wall... My target will be the 51Hz Mode in this case. I still think about a good material for this... I am looking for something with 5kg/m^2 or above...I think it could be hard to get MLV from a local dealer(Austria)... i have to look more on this and alternative materials... I think it is also good to have measurements with the brick wall built up to find out the exact room mode frequency

diffusion couch problem

-maybe i make something like a variable part in the QRD where you can replace this part with a broadband absorber. or are there other ideas to solve this?

speaker wall construction

-is there something i should change or do you think that I (my friends) can start building up the brick wall in the meantime?

Thank you very much