I believe that the amount of stuffing material is as important as the nature of the material used. And it is not easy to find the right combination.
Beside stuffing material and amount, I have found a third factor that influences the sound coming out of the midpanel: the way the fibrous material is packed.
I tried two different packing directions:
1) perpendicular to the driver , this is the usual way, the stuffing material is packed at the bottom of the box (assuming that the speaker is laying on the floor with the driver on top).
2) parallel to the driver, the stuffing is packed against the lateral side of the enclosure.
(1) and (2) sound different. (2) has more brightness than (1).
These results apply to Miraflex. My guess is that this is also true for other stuffing materials.
The only explanation I could find is that that in the first case, the fibers are packed in the same direction as the sound wave coming out of the back of the mid panel.