@dynamiclinearity @bdp24 - as @mijostyn pointed out, the SPL dropoff with distance has nothing to do with the speakers being planars or dipoles.
Line source speakers produce a cylindrical wavefront (whether they are dipoles or not), which has an SPL falloff of 3db with each doubling of distance. A point source speaker (most conventional speakers) produces a spherical wavefront which reduces SPL by 6db with doubling distance.
This is why you'll see a stack of speakers on stage for large rock concerts, creating an effective line array so that the audience sitting close to the stage doesn't get totally blasted out when the sound is loud enough for the audience in the rear.
Using a dipole sub vs a conventional sub isn't going to affect the SPL falloff in the bass, but since the wavelengths are so long, the sound pressure at any point in the room is affected as much by room modes as anything else.
That said, I do prefer the sound quality from dipole subs (for music). I think they sound more natural and integrate more seamlessly with dipole main speakers.