I don’t yet know anything about the new ET woofer system for the LFT-8, but unless it’s a dipole design I can’t see how it will be as good a mate as is any dipole sub (including the Rythmik Audio/GR research OB).
Magnepan is working on a "concept" model which will include dynamic (cone) woofers of OB/dipole design to augment their well-known magnetic-planar midrange/ribbon tweeter panels. Their MG30.7 model uses very large panels to reproduce bass frequencies, those panels sharing with OB/Dipole subs the same acoustic properties.
I own a pair of the Magneplanar Tympani T-IVa loudspeakers, which is basically a 30-year old version of the MG30.7. The Rythmik Audio/GR Research OB/Dipole Sub comes closest to equaling the outstanding sound of the T-IVa bass panels as I have yet to hear. Remember when Harry Pearson created his own "Super Speaker" by using the Tympani bass panels with the midrange/tweeter panels of Infinity?
OB/dipole woofers share with dipole planar loudspeakers the same drop off in SPL output as speaker-to-listening position distance changes. Omnipole subs (sealed and ported boxes) do NOT, so the sub/speaker balance is "correct" at only one speaker-to-listening position distance. That is imo a major flaw, one inherent in the mating of dipole loudspeakers with omnipole subs. Dipole subs also load the room the same way dipole loudspeakers do, very differently than boxed loudspeakers and subs. Mating omnipole subs with dipole loudspeakers is doomed to failure: it can’t be done, at least not to perfectionist standards.