Today I spent ~5-1/2 hrs. trying out a few combinations of spots for my subs:
1. Front left inside (between left main and CC) and back right corner (in front of the half wall).
2. Front left corner and front left inside.
3. Front left corner and front right inside.
After all that, the best spot for the subs turned out to be....their usual spots: Front left inside and front right inside.
I get the best overall FR there, flat to - but, dammit, never below - 17Hz.