here's a graph showing specific null frequencies I found when I had just 1 sub in the corner, measuring along the length of the room (the longest dimension)...they occur at specific locations that just happened to be in bad seating locations.
length nulls.jpg
here are the height nulls I found, when the sub was on the floor, and the mic was at 51, 57, and 58" from the ceiling.
height nulls.jpg
the width nulls didn't really line up with the theoretical predicted nulls at locations expected, these occured at 96 & 99" from the south wall, along the east wall. the sub was at the southeast corner.
width nulls.jpg
I tried putting the subs at the 1/3 & 2/3 locations along the length at first, it didn't seem to help much. Then I tried a sub in the corner and one in the middle of the length to combat the huge 19 hz hump. Then I tried one in the middle of the east wall, and the other middle of the bottom...didn't like that.
I might also try 1 sub at 96/99" along the east wall, and the other in the middle of the length to see if that gives even flatter response...I had to pull the east sub away from the corner, and the couch away from the wall or there was a major hump around 44 hz...
It's really hard to explain all the different combinations that I've measured, but I thought I'd try to explain a little in case others were wondering...