cab's ribbon speaker idea

..I think at some point you have to consider DIY'ing a multi-driver single-panel *DML for the mid.s and upper bass; it's not a dipole (rather a bipole) but often sounds similar and I remember having seen some designs getting good non-linear results (though most of the time it's pretty horrible).

*and distributed mode loudspeakers can sound really good.

Definitely a design that requires a LOT of experimentation/measuring, but CHEAP! 😎
Unfortunately measuring something like this is extremely difficult for non-linear results (..nearfield and DML's is tough).
 
The Bass on my Tympani T-IVa was incredible...those were my all time favorite speakers...but they were huge and had to sell (I had three pairs of T-IVa over past 15 years).

I now have Tympani T-Id bass panels that I will be mating to a ESS Heil Air-Motion Transformer...crossed at around 1khz...gotta sell my Quads first so I have the room in my office...

They will still a bit large, they will come in 2/3 the size of the T-IVa...but the T-Id lack the bass extension of the T-IVa bass panels...