Speaking in general terms, this is why I really question the value of having a subwoofer as a hysical part of the mains.
When a speaker is positioned for the best imaging, it's generally not in the same position that would maximize a sub's contribution to the over all sound. You wind up, at best, with a compromise.
When the two are separate, one can concentrate on placing the mains for maximum effect and then take on the onerous task of positioning the sub for less room interaction, nodes and other things that can plague those pesky low. low frequencies and can make then overpowering, non-existant, turn them into mush, or any combination thereof.