BL is referred as force factor or motor power. The discussion about motor strength is most often about BL²/Re. You can read the definition on TC Sounds glossary.
Using BL^2/Re, the combined motor strength of the Swarm is 4 x 88.5 = 354 newtons squared per watt, which is a unit that I have a hard time visualizing. Anyway thanks for pointing this out, I think BL^2/Re is indeed a better yardstick.
Data-bass.com provides a sort-able list of BL²/Re of all drivers tested.
Thanks for the link. Looking at total BL^2/Re, the Swarm still has more motor strength than all but one uberwoofer on the list, a 21", 50-pound, neo-magnet specialty prosound woofer made by B&C that has an Re of only .7 ohms. In particular, by this metric the Swarm outscores the best TC Sounds 18" woofer, 354 to 229 (the TC Sounds is the highest-BL^2/Re home audio subwoofer driver on the list). I'm NOT picking on TC Sounds - I think they make the best subwoofer drivers on the market; rather, I'm ganging up on their big boy with my four little guys. When I did a custom 16-Hz Swarm for someone a few years ago, I used TC Sounds woofers.
What is additive with drivers is displacement...
If we increase displacement by doubling cone area (and moving mass) without also doubling motor strength, we end up with a boomy mess. So in that sense BL (or BL^2/Re) is additive alongside displacement, if we want to retain the same basic transfer function.
If we add up all the BL in the Swarm and divide it by all the Re, we see that Qes hasn't changed. But we displace four times as much air for the same amplifier input, relative to a single woofer. Where does the additional power needed to do that come from? From the permanent magnets in the motors.