When I considered push-pull for seven π cornerhorns, I realized that the top of the line model already has a mechanism for reducing distortion caused by flux modulation, so push-pull drive wouldn't be as effective for it. The woofer used in the Professional Series seven π uses a shorting ring that is effective down to below 100Hz. Shorting rings begin to lose effectiveness at subwoofer frequencies, so I'm not sure they make sense for dedicated subwoofers but for the bass bin of a cornerhorn, flux-stabilized woofers are perfect.