@markus I skimmed both papers, and I have two questions:
1. For the Davis paper, could one achieve a relatively similar result by using two cardioid speakers pointed straight at each other? That basically seems to be the radiation pattern shown. "The loudest axis points not out into the room but at the other loudspeaker..." The contrast between the loudest and softest axes seems to depend on the distance between the loudspeakers.
2. The the PI-stereo paper, the apparent positioning of the loudspeakers relative to the wall behind it was straight ahead, as shown in Figure 1. Figure 12 again seems to demonstrate a radiation pattern primarily directed 90 degrees from "out into the room." The ideal radiation pattern seems to have an almost warped bipolar quality to it with cancellation at about 200 degrees and some dampening (~5 dB) of the backwave. Could one achieve a relatively similar result by using two curved panel loudspeakers (Martin Logan?) pointed straight at each other, with an absorption element like a TubeTrap or acoustic panel positioned directly behind (relative to each panel) each speaker?