They are using passive radiators in a lot of their models so similar concept.
Iāve never had subs, but I was once told the human ear can not differentiate sound direction below 50hz so in theory 2 are not needed.
Again, recommend reading Floyd Tooleās āSound Reproductionā that discusses speaker/sub/room interaction. Toole worked decades for the Canadian Research Council who helped domestic speaker companies with testing, then went to work for Harmon International. Heās very respected. All this discussion would be cleared up with a reading of his book (3rd edition is the best/latest).
One of the analogies from the book that I value is a bathtub with 4 inches (10 cm) of water in it and moving your hand in the water from end to end (behaving like in-room bass waves). As the waves hit the end of the tub they bounce back and hit the next wave, causing either canceling, reinforcing, or otherwise interference. Those peaks and dips is what bass from a single source is doing in your room and vary by frequency and location. It has nothing to do with directionality of bass frequencies.
One of the theories for placing subs in-room uses Fibonacci ratios. Place the first 13 inches from a corner, the second 21 inches, the third 34 inches, and the fourth 55 inches from the corner. Place one say 6 feet off the floor and try reversing phase on one of them. All this to randomize the bass generating locations and thereby minimizing peaks and dips which can be +/- 30 dB.
Stereo speakers are set up side by side, usually pointing lengthwise into the room, so using the bath tub analogy bass peaks/dips are doubled, the deeper the response the worse. Thus my preference for monitors, cut off at say 80 Hz, the THX standard and then using 3 subs carefully positioned.
But room shape/size is also hugely important, IMO the 2nd most important system component. Bigger is almost always better, but shape is more critical. Room dimensions should NOT be in simple ratios of each other (1:1, 1:2, 1:3, etc.). One of the best set of ratios is the aforementioned Fibonacci ratios (where the 2 smallest room dimensions add to produce the third dimension). Ancient Greeks called 5:8 the Golden Ratio, a Fibonacci ratio, which western eyes (and ears) find it a pleasing proportion.
In theory being the key phrase. Most folks start out with one, then get another and go, āOh. Now I get itā. Helps it be balanced in the room, require less output from each, can be fed separately from the L/R channels, so stereo is maintained down there. You can cross over at 35 Hz, and having one sub louder than the other steers the apparent balance of the whole image that direction.
Thank you all for providing suggestions and comments to my questions, very much appreciated. I just saw that Paul posted a video āIs stereo subwoofers betterā at his Ask Paul Videos.