Nuance - I did try reversing phase as an earlier solution, but the change did not affect the peak. Also, I'm using a Denon 3312 for the mains and it doesn't have an adjustable crossover (that I'm aware of). So I'm stuck with the 12db slope for now.
I agree that the room mode can be EQed, but since it's caused by the mains, my sub EQ is of no use. It's already set to cut the maximum amount (15dbs) at 63.4Hz. I will try using the graphic EQ that is built into my AVR to see if that can help. I was able to confirm that when Audyssey is set to manual mode, it simply acts as a graphic EQ and does not apply any of the Audyssey corrections from the measurement procedure. If this works out I'll repost the frequency response.