I designed and made my own treated listening room recently. Mainly absorption ranging from 50cm (effective) treatment in the corners to 10cm at the first reflection points (including the ceiling). The balance was perfect apart from the window which was creating a node in the vocal range. Treating that with absorption undid all the good from the other panels and it suddenly became dead sounding. To overcome this I designed and made 3 quadratic residue diffusers based on prime 7, which did the trick. The result is absolutely sparkling. There are no frequencies fighting each other, every one separated and clearly audible and the sound staging is immense. I can have instrument playing right by my ears whilst others are ‘seen’ way beyond the front wall.
By making my own I was able to check that each one made a positive improvement to the room before making more. Yes there is a lot of theory and opinion out there, and I read an awful lot before starting, but you will only tell whether it’s worthwhile is by giving it a go.