From an old thread on Oade board.........
xy and other coincident configurations align the capsules so that all
sounds, direct or reflected, reach each capsule at the same time.
this gives you perfect phase coherency. this phase coherency tends
to make the midrange and high end sharper and more defined, but what
you perceive relatively is less low end.
any near coincident configuration results in phase incoherencies.
phase incoherencies give you less midrange and HF energy (or more
low end energy, in relative terms), and are interpreted by the ear
as spaciousness or diffuseness, aka boominess.
callery