Duke, agree with everything you say above. For clarity, I didn’t describe an OB/Dipole sub as a line source, rather that such a sub behaves like a line source loudspeaker in the matter of SPL drop off as listening distance changes. Even the mighty Infinity IRS suffers from that weakness. It uses a column of woofers (servo-feedback controlled) in a sealed enclosure for bass, a large "wing" with line sources of EMIM’s and EMIT’s for mids and highs. Again, the balance between the woofers and the m/t drivers changes as listening distance increases and decreases.
Your idea of reversing the polarity of one or more subs in an DBA is a great one. Danny Richie displays at shows with a pair of OB/Dipole subs at the loudspeaker end of the room, a pair of sealed subs at the other, their polarity reversed.
Rythmik’s Brian Ding prefers sealed subs to the OB/Dipole, liking as do you the pressurizing of the room the former provide. He finds the OB to sound too lean, without enough weight. Others view that weight as too plump, or fat. Each to his own! To my ears, the OB/Dipole Sub sounds very much like the bass panels in the Magneplanar Tympani bass panels (which I also own), still considered by some as the best reproducer of instruments like bass drum, upright bass, cello even, the lower piano and organ registers, etc., ever offered to the consumer. Some feel the same about the bass produced by the Apogee full range ribbons, which I've never heard.