My understanding is that the use of polyfill or not depends on the woofer and port. In some cases the system may exhibit some peakiness in the bass area. Using polyfill reduces this issue. In some cases however, the bass is just right w/o any fill, and adding fill when not needed will only make the system loose bass w likely no benefit. In other words, using too much fill or fill when is not needed may essentially castrate the rig. I would forget about the back wave talk and all that. The box is more of a pressure chamber in my mind. And pressure goes on all directions inside the box. If the system is experiencing cancellation, it will show in your measurements.