Just doing some reading, it looks like there is a lot of stuff running 20hz and below. I may have to plug up one of those ports.
Classic wisdom is one in center front, one in center back of the room, or opposite midwall on the sides.
Home Theater Multiple Subwoofer Set-Up & Calibration Guide — Reviews and News from Audioholics
Figure 1c. 1/4 W Placement
Although this isn't spelled out in the CEA recommendation, Dr. Toole references it in his book as a good solution for two subwoofers but suggests additional subwoofers may be needed. In my experience I've had excellent results placing two subs against the front wall at locations of 1/4 the room width. I've had even better results placing two additional subs in a similar manner against the back wall. This configuration can achieve nearly as good frequency response performance as the 4 Corner placement with nearly as much bass gain as well.