I'm just catching up on this thread, but have a couple of comments.
For that null in the 90-110 Hz range, I would look to your M22s as they are contributing most of the sound in this range. Try playing with their positioning if you can.
For bass traps to be effective, you need them as thick as possible and you need as many as possible. For the corners I would go with 6" thick minimum, and would fill the corner solid as Chris suggested if you can. Make sure the stuff you use is a minumum of 3lb/cu.ft. in density or its useless for LF.
You need to fill all 4 corners at a minimum, and if you can, also treat wall/floor and wall ceiling intarfaces as well.
You might be better served to spend the money on a second sub. From reading about others here that have done this, it goes a long way in smoothing out the frequency response.
