Progress report:
After taking everyone's comments into consideration I decided to tackle my back wall. I have the luxury of not needing to contain the LF in my room. Low end leakage doesn't bother anyone and there is no significant source of LF outside of the room to bother me. I removed the inner gyp board wall and added a huge amount of porous trapping above and to the sides of my rear diffusor. The combination of more LF leakage and trapping (the trap is now about 5' deep at its deepest point above the rear diff) has made a huge difference. The 60Hz bump is gone and the 105Hz bump is much better. No resonators needed!
The next step was to work on speaker placement optimization. With all of the LF changes, the apparent LF in the room was decreased. I assume the modal ringing of the room had been creating a false sense LF power, even though I knew it wasn't right. Now that the ring is gone, the LF is more even, but not as well reinforces as before.
Small speaker placement moves can make surprisingly large responses changes. After a few hours of moving the big PMCs a couple inches at a time, I found what seems to be the flattest LF response with the fewest peaks and dips. The RPlusD software (the update to ETF5) made the process of comparing many speaker locations a snap. If you need a measurement tool but don't need to make a career out of it, RPlusD seems to have everything you need.
I've been listening all day and I have to say I'm extremely happy with the results of a weekend's work. Thanks to everyone who encouraged me to do it the right way!
GR