We have four stacked JBL subwoofers behind the screen. Subsonics traveling through the auditorium is NOT a problem, it's one of the goals. 
By "four stacked JBL subwoofers", I'm sure you mean four factory prepared and loaded enclosures, each with a couple of eighteen inch or fifteen inch drivers it them, right?. The best in the business, until you get into some of the custom engineered cabinets available from Meyer Sound Labs, and a few others (but John Meyer is an old acquaintance, who I respect)
My subwoofer system is JBL based also, but I have one custom self-built enclosure which uses four vintage JBL fifteen inch drivers, in a ported alignment called an Extended Bass Shelf (referring to a graphic of the frequency response it reproduces) which I designed to resonate at twelve hertz in one configuration or sixteen hertz wide open (the tuning I use, now), depending on the rooms characteristics and how the positioning of the enclosure interacts with it. That enclosure handles the frequency range from about sixteen hertz to twenty eight hertz, where another two fifteen inch drivers in an enclosure, also of ported alignment design (constructed according to the JBL factory recommendations for the specific drivers I use) takes over and reproduces information up to about one hundred twenty five hertz. I use a mono-strapped vintage McIntosh MC2500 (one thousand watts, very conservatively rated) on the EBS enclosure and about five hundred watts on the upper sub cabinet. Not nearly enough for a theater, but plenty for my sixteen by twenty four foot living room. The Crossover is done electronically by a vintage Crown VFX-2a, set for twenty eight hertz, that has been modernized with hot-rod capacitors available from Japan's audiophile industry.
The receiver is a Yamaha one hundred watt per channel (Typical) and (5.1 ... I haven't gone nuts, yet with the seven and ten surround goofiness) surrounds are vintage Yamaha studio monitors from the early eighties era, designed to produce the "East Coast Sound", promoted by Acoustic Research. "East Coast" sound was intended to be more of a "Flat Frequency Response", where a "West Coast" sound, promoted by JBL and Altec was intended to emphasize the high and low frequency extremes of human hearing. I prefer a flat response with a substantial reinforcement of the low frequencies, myself, so I mix the two theories. Movies kick ass at my house, compared to what somepeople listen to them with.
But Peter, that's not an audiophile rig at all - it's just my souped up movie rig for the living room. (I have a very understanding wife) My so-called audiophile rig consists of only a few items and is quite minimalistic to the extreme. My tube based, one watt per channel, audiophile quality, amplifier only has eight silver solder joints in the signal path! Sweet!!
The reason I offered you Linkwitz's site was that he is a true scientist, who has published his life's work, freely, on the internet. You don't have to spend much money to enjoy phenominal sound, using the work he has done. Of course, if you can afford to buy his equipment, you won't need to learn much about his work.
