Chess, since the placement question has come up, let me say that, looking at your setup, you certainly wouldn't want less separation between the M60s; if anything the left M60 might be moved a bit wider. On the sub, since right in the corner is nearly always the best location for both strength and smoothness of bass(since it's about equidistant from floor and both walls), it might be worthwhile to try the corner behind the right M60. If the bass is louder or even "boomy" there, the sub level should simply be reduced.
