i only used my homenmade devices and various materials sandwiches but it is my experience that each material will absorb in his own way some frequencies more than others and will couple with others materials in his own way too ...
I selected some various materials for their "timbre effect" ...
And the tuned damping load over the speakers too was very important for details by eliminating or decreasing resonance...
Then i think this sentence about different footers must be right :
I find each have their own impact on tonal balance.