Sorry, I mean a soundfield mike. Capture both all 3 volume velocities and the pressure at two points about 15-20 CM apart, on a line parallel to the line between the speakers.

Remember that in any point in the atmosphere, there are 4 variables, 3 volume velocities (which can be represented in two ways, but velocity is easier to interpret than polar for what I'd care about) and the pressure. The pressure at one point (that's what an omni does) is only 1 of 4 variables at that point.

With all four variables you learn a great deal about what reflects where, what room modes look like, what various time delays are, and so on.

Remember that in any point in the atmosphere, there are 4 variables, 3 volume velocities (which can be represented in two ways, but velocity is easier to interpret than polar for what I'd care about) and the pressure. The pressure at one point (that's what an omni does) is only 1 of 4 variables at that point.

With all four variables you learn a great deal about what reflects where, what room modes look like, what various time delays are, and so on.

I looked into soundfield microphones and they look like an excellent tool for room acoustics evaluations.

I'm actually far more interested in optimizing loudspeakers for favorable speaker/room synergy. For example, regarding room acoustics you wrote that "dead is my preference." This implies that the less energy a speaker puts into reflections, the better. Which in turn implies that a highly directional speaker is preferable, especially for rooms where sufficiently aggressive acoustic treatment to render the room "dead" is impractical.