First comment -- wow, that's a lot of testing
I haven't plotted response as carefully as you but I remember getting readings with almost 20dB swings. Your room might be a bit worse but not much worse.
Other random comments :
How big is the room ? I'm guessing one dimension is about 11 feet ?
Sorry if I missed this, but was the graph you posted with the meter as shown in the picture or in the sweet spot ?
Was the blanket positioned to intercept the first reflection off the walls to your listening position or was it really close to the speaker ?
Have you had a chance to convert the peak & valley frequencies to distances and go looking for those distances ? I was just working the numbers in my head during a particularly dull meeting today and if I didn't screw up you're looking for 4-1/2 foot and 11 foot distances, either wall to wall or front of speaker to wall.
IMO you definitely did the right thing measuring the speakers independently. You can average the readings and plot what you would get with both speakers turned on, but you can't separate the plots if you measure together.
