It's been said that restive broadband bass traps can improve with the addition of plastic film sandwiched between layers of rigid fibreglass. Here is a test to demonstrate. In this case, a low profile bass trap was used over the front wall, 0.3m out from the wall. Compared is the baseline (no traps - grey), raw traps (blue) and traps with plastic added (red).
These measurements were taken by Mellow Fellow and posted on the StereoNET forum. You can see the biggest improvement comes from adding the traps, but the film also increases the damping effect slightly.