Stephane, another reason for keeping the same phase setting on your two subs is to maximize the smoothing of room modes. When two subs are in diagonally opposite corners they both excite the room modes for length and width to the maximum, but the positioning results in a partial cancellation of those modes because each sub's output is of opposite phase when it reaches the other corner. So, to allow this effect to operate, the subs should start out with the same phase setting.

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Enjoy the music, not the equipment.