Hmph. And that really works such that the sound difference is really noticeable?
Passive radiators work in a manner similar to a bass-reflex port. In a reflex port, there is a mass of air suspended within the port tube that 'bounces' on the volume of air in the box and absorbs the acoustic load at low frequencies. A passive radiator uses a mechanical mass and a mechanical suspension along with a large diaphragm to achieve the same result, except with less compression and no wind noise because the pressure inside the box acts over a much larger surface area to get the mass element 'bouncing'. At low frequencies, the passive radiator is absorbing the acoustic load from the woofer and radiating that resonant energy into the room, which means that the active driver doesn't need to work as hard.
Brian Cheney was fanatical about bass quality and quantity, and he used the slot-loaded passive radiator configuration in virtually all of his floor-standing speakers and subwoofers. The slot-loaded design causes the passive radiator diaphragm to work against an acoustic resistance (the slot), further damping the resonant action of the passive radiator and enabling transient response that is somewhere between a vented box and a sealed box. The amount of putty in the center of the passive radiator can be tweaked to adjust the level of bass damping further.
I looked into the slot-loaded passive radiator configuration some time ago, and what I've told you is as much as I've been able to find out, since Brian kept this enclosure loading method fairly close to the vest. It is a minor variation on conventional passive-radiator speaker designs, and it was never patented.
Also, regardless of what the Parts Express web site says, you can use those re-foam kits to re-foam this particular passive radiator. However, it might be better to let their Speaker Repair Center do it if you aren't sure of your refoaming skills. These speakers deserve to be treated well.
