DI is the difference between On-axis or the Listening Window and the Sound Power. Some manufacturers use On-axis and others use the Listening Window. I am assuming Axiom uses the Listening Window because that is what they publish. Recognize however that it's up to each designer to interpret the results as they see fit. So Axiom may use the On-axis curve to tailor the DSP filters in order to make the Listening Window "better".
"Omni-directional" is only in the context of the curves I describe in my poem above. Don't interpret "omni-directional" to mean you can sit directly in between the plane of the speakers, or behind them, and experience Nirvana. You have to be somewhere between the speakers and at a distance where the direct and reflected sounds can be heard (diffuse sound field) which may be 12 feet or more away depending on the room and the distance between the speakers. On this point, Ian has said measurements are taken circumferentially. I think this is well-beyond the SAE spinorama standard which includes the angles in my poem. It's just another example of Axiom wanting to characterize their speaker system as much as possible so they can eke out every last bit of performance.
The DSP compensates for the delay between front and rear drivers.
The Sound Power is indeed the SPL in dB at 1 meter away. I don't know how far the mic actually is.
