not sure you answered my question, what mode are you using: direct, stereo, PLiix, etc? My m80's are seperated about 9-10ft in the theater room, front row is about 8-9 ft back, and 2nd row is more like 13-14ft, and there are no issues. The m60's should be as forgiving as the 80's.
my guess it is a room issue, what is your floor and walls like? if you have hard surfact floor, try throwing a carpet piece or something between you and the speakers.
M80s-VP180-QS8s-EP600-2xEP350 Denon3808 Outlaw7700