The XS15s are currently postioned; one up front, between FR and TV, and one in back left corner (phase set to ~90 degrees). I seem to get better coverage and smoother bass than when I had both up front. I am also currently awaiting my new USB mic to get more in-depth with REW, so things will most likely change after I get some good measurements.
I'm using a Denon 2113ci and external EQ (BFD) is definitely on my list (if needed). My plans after I get some good measurements with REW are to (1) optimize sub placement,(2) apply acoustic treatments, then (3) EQ, with finally (4) more subs only if needed.
I'm just kind of thinking ahead, and more subs may not even be needed...guess I'm kinda jumping the gun really. I need to get solid measurements of my room before I can start doing anything really.