A push-pull approach does possess measurable distortion lowering advantages in small sealed subs, and especially IB subwoofer systems.
As one can imagine, and similar to most all things subwoofer related ... there's much myth and misunderstanding about such techniques. Take any normal, double-driver, small sealed sub, ie., the monster JTR Captivator S2;
Obviously, both cones work together to create waves of bass. The cones are both mechanically in phase, and electrically in phase. To implement a push-pull approach, flip one driver around backwards,
and flip it's polarity too. As one can see, the drivers are still working in concert with one another, and it's sensitivity and maximum output haven't changed at all.
What has changed is the manner in which the driver's acoustic energy sums together. The result is odd ordered harmonics are cancelled out. Magnetically, there exists a small asymmetry between the forward stroke, and the rearward stroke of the cone. Designers try to keep this as small as possible, but it's just a product of the magnetic motor/voice coil approach of dynamic drivers.
Essentially, any anomalies in the driver's cone, surround or spider, or magnetic properties through the stroke, are simply canceled out by the flipped driver's opposite characteristics.
Since the coil is moving, and the permanent magnetic element isn't, there's some non-linearities over the course of the stroke. The specifics are beyond the scope here, but combining the even ordered nonsymmetrical non-linearities (including the aerodynamic of the structure/basket, the spider, surround), then entirely nulling them by flipping one driver's orientation and polarity, is a distortion lowering technique that has merit.
The odd ordered harmonics remain, yet the even ordered are lowered. A small sealed, like the JTR CapS2, benefits, however the air volume loads the drivers, and although there's benefits, they're somewhat lessened. That said, conversely ... an IB subwoofer system utilizing a push pull approach fully benefits from this technique. The drivers need to be relatively close to one another, but given the sizable wavelengths being produced, typically this isn't an issue.
Now, why don't more mfrs make their subs this way? It's difficult to implement the reversed driver. Also, although THD is lowered, the remaining odd ordered harmonics are oftentimes the most objectionable subjectively, the even being somewhat more tolerable. THD is lowered in push pull, period. Some of the finest studio monitoring systems in the world, the TAD based Kinoshita systems (Rey Audio), can be upgraded with their sub system, which is push-pull. They've been offering 9hz extension playback since '91.
some Kinoshitas demoed, for sale. They use TAD drivers, if you've never had the pleasure to experience TAD, they're superb.
I hope this helps
Thanks
btw ... found a push-pull pro audio, vented sub by EAW;