For this project, I actually prefer a vented enclosure. The reasoning behind that is that these speakers will be used for home theater. Generally a home theater receiver only has two settings - large front speakers and small front speakers. These speakers will go deep enough to be called large speakers, but the receiver will send deep bass to the subwoofer so I want the low frequency response to be as flat as possible. The other possible, calling them small speakers, will usually engage a high pass filter (crossover). To make that filter work appropriately it is best to have flat low-end response - again predicating a vented enclosure.

The last reason to use a vented enclosure is pedantic - I think it's important for purposes of the tutorial to use a vented enclosure.... so

We bring up the Vifa enclosure and select the Calculate Vented... command. This brings up a dialog box that lets you pick between three different alignments (BB4, C4, and QB3). All three alignments work satisfactorily for vented boxes - and other alignments work as well, but these three are good starting points. Interestingly, C4 and QB3 are identical for a driver with a Q of 0.4, which the Vifa is close to, and hence there won't be much difference between these two.

Comparing the two - QB3 has a shorter port and bigger box, while C4 has a longer port and smaller box. Their response curves are very similar to each other.

Here's a comparison of the C4 alignment (with a 0.3 ohm series resistance) with the butterworth sealed box alignment. The butterworth alignment results are in red.