I started out by deciding on a ported enclosure for the woofers. The woofer Q (spec is .34, measured was .44) is suitable for a ported design (optimal Q is about .32, Q's over .5 are too big for a reasonable ported system). Also, a ported design is nice and flat in the low end and has good power handling. In most cases the system will actually have a receiver generated crossover at 80Hz or 100Hz (a typical dolby digital receiver) so the flat response and good handling work well with the receiver to get a total flat system response with good handling. In any case, most movies will have very little deep bass in the non-subwoofer so our very deep bass handling (which suffers in a ported system) will be irrelevant.

We now create a new enclosure, put the PL18 in the enclosure, and use Speaker Workshop to calculate ported designs.

Here's a chart showing the ported versus sealed enclosures for the PL18 we've tested. The sealed response is good, but by using the ported response we will get better system integration. For the ported analysis I assume a series resistance of 0.4 ohms (a large series inductor) and a port diameter of 2". These results are from Calculate / Vented... in the enclosure menu.

Here are box results for two runs of the Vented... dialog. As you can see the QB3 and C4 alignments are very similar and both give good results. I plan on using the QB3 alignment because I think it sounds slightly better than a C4 alignment but in this case the difference is trivial.

I decided to place the port on the side of the enclosure. This allows for easier room positioning, reduces midrange feed-through of the port, and generally works better than any other port location.

I decided on a dual-woofer system (the D'Appolito configuration) because it has good acoustic properties for home theater and it can handle lots of power.  Usually the woofers are in parallel to minimize issues if they aren't matched. Since I can hand match the drivers I put them in series so as to double the impedance seen by the amplifier, making the speakers very easy to drive by a typical 6-8 ohm rated amplifier.

Here's a drawing of the enclosure (click it for a full-size view).

If I had it to do again I would make the woofers closer together. I left space for the shelf to be misaligned, but in practice the construction is so high precision that very little clearance is needed for the shelf and the lower woofer could be moved up (although this would increase near reflections).