Hi McJenn,
Unfortunately the most meaningful information that could provide some insights into different layering arrangements would be any differences in your “symptoms” and sleeping experiences between different combinations. It would be very unusual that someone doesn’t notice any differences between different combinations although it’s possible that this could also be “pointing to” other issues that may be contributing to your back pain that may not be connected to your mattress.
Without knowing more about the differences in your sleeping experience between the different layering combinations I don’t really have a frame of reference or enough information to be able to provide any insights or guesses into the type of changes that may be worth trying.
It may be worth trying the XF/F/M again if you can convince your hubby to go along with it for a few days.
It may also be worth trying a medium layer on top with your soft layer underneath it (and then the firmest layers underneath them).
If your mattress is too soft then some of the suggestions in post #4 here may be helpful as well.
Phoenix