Interesting Louis.
I'm just clarifying in my mind how one driver can be used satisfactorily in different configurations. I can appreciate that doubling the driver would increase efficiency by 3dB (perhaps more due to coupling). I suppose that one of the difficulties we all face is the lack of standards for efficiency, i.e., test conditions (anechoic or using room gain), bandwidth, and variation from reference level (+/- 3dB, +/- 1dB).
If we assume that +/- 3dB is an acceptable frequency response, than a -3dB level below 300-500 hz with a single RS5 driver really isn't that big of a deal. Logically, adding a second driver below 500hz would contribute an added fullness without seeming bass-heavy. Along the same lines, adding a second driver off axis, either facing upward (omni) or rearward (bipole) would add additional bass energy with higher frequencies rolled off off-axis. It would appear that all three methods of adding a second driver effectively increase bass-midbass frequencies by 3dB or more without an appreciable increase level above the midbass. In effect, we get the benefits of a smaller driver in high frequencies along with the ability to move more air at lower frequencies.
I would argue that the best choice among these three dual-driver approaches would depend on the room and speaker positioning.
1. 1.5 way - forward radiation pattern, minimal room boundary interaction. This speaker can be placed close to the front wall. This boundary positioning will also provide some additional bass boundary gain with the rear ports coupling to the wall.
2. Omni - forward and upward radiation pattern. The upward-facing driver adds bass and upper frequencies are attenuated by being off axis and delayed by ceiling bounce. This speaker can also be placed close to the front wall, but relies on a relatively high ceiling to get adequate time delay from the upward facing driver to avoid smearing.
3. Bipole - forward and backward radiation pattern. The rear-facing driver behaves similarly to the Omni driver described above, but uses the front wall instead of ceiling for reflection. This speaker must be placed a significant distances (>1m) from the front wall. This would be terrific for near-field listening in a small to medium sized room.
A single driver RS5-based speaker can be satisfying as well. I wold posit that when positioned near a room boundary (such as the front wall) the midbass will be enhanced, providing a fuller sound. Likewise, a wider baffle such as the original Super 3 or the Super 3XRS could provide similar midbass enhancement.
If we assume that the single-driver speaker is shelved 3dB down below 500hz or so, its not the end of the world. It may be a bit leaner sounding (this corresponds with my own experience with my Super 3's) but still adequate in the midbass. I would argue that for music with a lot of female vocals, the dingle-driver version is absolutely perfect:
http://www.audio-issues.com/wp-content/uploads/2011/07/main_chart.jpgMy gut feeling is that perhaps the ideal expression of the RS5 is an omnidirectional XRS. With a slight backward tilt, the omni driver is firing at the boundary of the front wall and ceiling, providing the maximum delay while allowing a closer distance to the front wall than a bipole.