A 3dB boost at 20Hz is NOT a shelving circuit. A shelving circuit is a progressive low-pass compensation filter, not a static figure (6dB/octave, the boost therefore increasing as frequency decreases), which exactly compensates for the 6dB/octave front-to-back cancellation inherent in dipole speakers and woofers. Without it, an OB/Dipole sub WILL exhibit declining output with frequency, and of course not perform optimally. That's just a poor or incomplete design, not an insolvable weakness in the design. ALL designs present their own challenges to be solved by a speaker designer.
Siegfried and Brian Ding did just that, and their OB/Dipole subs do NOT exhibit the failing of a missing bottom octave. Linkwitz didn't chose to go with an OB/Dipole sub out of blind allegiance to some "pet theory". Give the a little more credit than that! He (along with Russ Riley) invented the Linkwitz-Riley filters, fer cryin' out loud!
The dipole-cancellation shelving circuit is well known to professional speaker designers, and is included in the DSPeaker Anti-Mode 2.0 Dual-Core. I wonder if the fact that both Gradient and DSPeaker are located in Finland has anything to do with that?