Lowther drivers, for example, radiate the high and mid frequencies from the cone, but the bass come out of a horn and are delayed in time and 180 degrees out of phase, as well as coming from the horn mouth rather than from the cone. So in a way, you have two frequency sources, with a crossover between them. The crossover is acoustic and needs to be tuned properly, and at the crossover frequency there is still potential for interaction of the two wave fronts. Bert Doppenberg's Oris 150 front horn for Lowthers needs a separate bass system, Bert uses bass horns with dedicated drivers. Another philosophy puts the crossover frequency high up, at 12 kHz, or 15 kHz, the higher the better.
Direct radiators have a harder time making that kind of bass.
Bass drivers with long throws and polypropylene cones are particularly bad. With the shorter travel, bass will become more controlled, crisper and more responsive compared to a long travel bass driver. Some paper cones can be stiffened up with varnish-like substances such as Damar. Good discussion on driver theory is by Bob Stout at LDSG . Rather, such drivers are systems of two or more drivers on the same motor assembly.