Best imaging comes from speakers that are placed symmetrically, as I'm sure you'd agree. But you've suggested running your mains down to 50Hz, so two octaves of the modal range are covered by them and only one octave by the subs. Since you prefer the modal range to be covered with speakers that are placed asymmetrically, how do you deal with that? After all, the worst modes in most average listening rooms are above 50Hz.