Boy, lots of stuff. Robert, as was said, the internal sub crossover has no effect on the mains, even if they're wired through the sub. It's simply a variable low pass filter on the sub below the selected frequency. Some subs have an entirely separate fixed(nominally about 100Hz)high pass filter on the speaker level outputs to the mains, but most, including the Axioms don't, and their effectiveness isn't held in too high regard. In general the mains connected to the sub speaker level outputs run full-range, so this isn't any different electrically than running parallel speaker wire to sub and mains .
Sean, I didn't consider your report to be criticism, because it isn't. I merely gave Nousaine's test results as relevant info that I was aware of; any disagreement is between his results and yours, I've done nothing like the experiments that you have. A point on terminology here is that the question isn't single sub vs dual subs, but rather dual mono subs vs dual stereo subs. As was said, Nousaine's blind tests for two mono subs vs two stereo subs indicated no benefit for the stereo configuration regarding directionality or spaciousness when the crossover was 80Hz or lower.
Another example of the "consensus" (which may be wrong, of course)on this issue is the often-cited
Harman report on multiple sub positioning, which used a mono configuration for the analysis and expressed doubt(pp.4-5)about stereo bass.
