@prof, I too have heard that "ghostly sound" from planars (most recently a pair of Maggie 1.7’s), but that can be and often is a result of comb-filtering caused by the back wave of the speaker bouncing off the wall behind it, meeting up with the front wave, and causing frequency-related cancellation. Planars are less effected by sidewall reflections than are point source loudspeakers, but much more effected by those from the front wall.
That an OB/Dipole sub cannot produce anything below 40Hz is complete and utter nonsense, assuming it is constructed properly---in an H-frame (GR Research/Rythmik) or W-frame (Linkwitz. See below), to prevent front-to-back dipole cancellation. Anyone who has heard the Gradient made for the 63, such as yourself, can attest to that fact. Robert E. Greene reviewed the Gradient/QUAD 63 combination in TAS, and reported no lack of bass below 40Hz. Same with those (such as myself) who have actually heard the GR Research/Rythmik OB/Dipole Sub.
Remember too that Siegfried Linkwitz employed an OB/Dipole woofer section in his outstanding LX521 loudspeaker, and it also had no problem reproducing the bottom octave. Does anyone really believe an engineer with as much knowledge of and talent at designing loudspeakers as had Siegfried would let one out of his lab if it had no bottom octave output?!
What IS true is that the output in general of an OB/Dipole sub is quite a bit lower than that of a sealed sub using the same driver (for instance, it takes four of the GR Research/Rythmik OB’s to equal the output of a single Rythmik F12G). But that is unrelated to it’s bottom octave---it is frequency-unrelated. It is for that reason some OB/Dipole sub owners use them in multiple sets, stacked atop one another. A Google Images search will lead you to pics of two, three, even four OB/Dipole Sub stacks. Not cheap, but SOTA never is. ;-)
On the other hand, the Gradient SW-57, made for the original QUAD, WAS deficient in the bottom octave. But then, it employed a pair of 8" woofers, and they can play only so low, whether in an OB/Dipole design, sealed, ported, or infinite baffle. The LX521 uses a pair of 10" Seas aluminum-cone woofers, the GR Research/Rythmik a pair of 12" paper-cone woofers designed by Richie and Ding and custom-manufactured for them. The same woofer, with an aluminum cone, is the one Rythmik installs in their F12 sealed sub (the F12G has the paper-cone woofer. Confused yet? ;-).