The subs should be placed in the room based upon the bass response in-room, not in relation to the Orcas. The frequency range they play in has wavelengths so long (10 feet and more) that the localization relative to the mains is secondary to whether or not you are setting off room modes. Try them in some spot, move them a foot and try them again. You will eventually find a spot where the bass is the most even throughout the range, with minimal boom or suckouts. Quite often a non-symmetrical arrangement of the subs will actually work better than a symmetrical one as each box excites different modes, thus evening out the overall response.
