I simply am reporting on a new speaker development.
Measured down to 21 Hz with what drop-off? I have trouble believing they are flat down to 21 Hz.
I had Martin-Logan Monoliths and Monolith IIIs for many years. I don't think I am particularly sensitive to the crossover discontinuity if it is implemented carefully.
To answer your question regardless of full-range driver design I like large cones brought in low, around 25 to 30 Hz. I would add REL No. 25s or JL Audio Gothams or 18" Velodynes or Seaton subwoofers or Wilson Thors, etc., to Apogees, Analysis Audios, Magnepans, Martin-Logans, Wilson XLFs, Rockport Altair IIs, etc.
(The surface area of the cones in the Pendragon equates to about 2 1/2 15" drivers per side.)