I moved this to the SoundSplinter forum...
Nice build Scott... and really nice extension!
So you moved them from being co-located to being symmetrically placed in your front corners and that improved your response. If you wanna try to get rid of that 40-60Hz trough... experiment with crossing over one of the units between 50-60Hz. I ended up having to move one of mine to the rear of the room and cross it over at 60Hz before I could get rid of a similar trough I had.