Thanks for the info Jim. My question about the naturalness of the bass comes from things I have read from Legacy speakers and from Magnepan both of whom claim that the greater the surface area of the bass reproducers the more natural the sound. Or is it simply a matter of moving air? The best reproduction I have ever had of an acoustic bass was some Maggie 1Ds I owned back in the early 80s. I mean if you played the LA Four direct to disc Ray Brown was in the room with you! Those speakers had two 7 sq. ft. panels for each channel just for the low end. But if the technology allows, can one manage the same quality reproduction from a single 10 or 12" driver?
The challenge is to move the appropriate amount of air in a manner that exhibits low distortion at the target SPL. This can be done in two ways.
First, you can use a large surface area with more limited XMAX requirements. Or, you can use a smaller surface area, but it will require a larger XMAX to make certain the driver is capable of the SPL required and won't bottom out.
As you can see by this graph simulating the SoundScape 10" woofer module, the driver (shown in blue) starts rolling off at about 80 - 90Hz and is 3db down at about 37Hz or so. Since the passives (shown in red) are tuned lower, their output is relatively low at that point and gradually increases as they approach their tuning frequency. Below 30Hz, most of the output comes from the passives. When you sum the output of the driver and the passives, you end up with flat response and an F3 around 21Hz.
If you start with a low distortion driver and do not allow it to exceed it's XMAX, you are half way there. The passives, then, need enough area and XMAX to avoid bottoming out at the tuning and SPL capabilities desired. If you can design a system that meets those criteria, the result should be clean bass with low distortion.
Note that at the passive tuning frequency, the driver's excursion is at a minimum. This is not necessarily intuitive, but that is the way the system works. So even though the system is tuned for a low F3, the driver itself never approaches its XMAX.
Keep in mind that, in this case, the lowest frequencies (those requiring the greatest amount of air movement) are handled primarily by a pair of 12" passives. If the cone area or XMAX of the passives were greater, perhaps only one would be required. But, in this case, two are required to reach the target SPL levels at the frequencies involved. You end up with two 12" passives which have 25% more cone area than a single 18" passive and they have plenty of XMAX to reach the design goals. So we're good to go.
Of course, this is an over-simplified explanation of what is happening, but hopefully you can understand it.