Jack, I really appreciate the lesson, and it does make sense now.
After switching things around and sending the same frequency to all subs, I got the following graph (no EQ applied).
BLUE is the 10" subs only. For music, I only turn these on.
GREEN is the 15" subs only. Never used by themselves.
RED is with all subs running at the same time. Used for movies only.
I think the red looks pretty good. Eventually, I may do away with the 15's and EQ the lower end for the 10's.
For what it's worth, in reference to just the 10's and the blue line; the drop from 25-35 is mostly due to that seating position. The 2nd row has more in that range and less around 50.
I feel a lot better about this and I'm now going to finish up the room and actually start enjoying it. A few more acoustic panels, and a slight change in seating should bring me more into the flatter curve, then I'll use the EQ if needed.
Finished product: This also includes my VR-3 main speakers that I made.