The reason for the passive radiators, even with the 10" version is because it is very difficult to get deep bass tuning in a moderately sized enclosure with out getting port noise and compression or being forced into an excessively larger port to avoid these things. And, if a system is being design to achieve the highest possible level of performance then we really do want o avoid compression and port noise if possible. This issue is even more problematic with high excursion woofers that move a lot of air, becasue they move a lot of air through the ports too, and this increases the requirement for a larger port. Passive radiators solve these issues all at one, and their only drawback is that they certainly cost more than a port tube. But let's face it, in a $10,000 speaker (just to arbitrarily grab a number) what's a $10 port, anyway. As long as the passive have enough surface area and linear travel - the reason we use two, as well as helping to keep the motion balanced in the enclosure - we can tune to the proper frequency and get full output in the low bass with no compression or extraneous port noise. It costs a little more to implement, but the performance gains are worth it. Don't believe the old myths that PR systems are boomy - that's just poor implementation.
Jeff
