One of my friends recently bought two Pyle Pro 18" woofers to build a sub, and I'd like to recommend that he use some sort of Pi implementation. His primary goal is maximum low end extension, followed by low distortion. Cabinet size is not an issue; in fact, anything smaller than a refrigerator might look too wimpy. :-) I'm guessing that a folded horn would be the best way to go, right? If he does that, is it possible for him to use both woofers in an isobaric configuration, or would he have to build two cabinets? I've kinda figured out how to use PiAlign and BoxPlot for bass reflex speakers, but I'm pretty clueless about using them to design folded horns, especially if I want to tune the sub lower than PiAlign recommends. Any tips? Here are the specs for the woofers:

Frequency range: 40-4,000 Hz * Magnet weight: 125 oz. * Fs: 27.4 Hz * SPL: 96.7 dB 1W/1m * Vas: 14.9 cu. ft. * Qms: 5.59 * Qes: .30 * Qts: .28 * Xmax: 5.5 mm

Thanks a bunch!
Mike Borzcik