Hi DMSexton47,
I agree that the firm would probably be a pointless endeavor and based on the odds … the medium would probably cover the single largest part of the bell curve that would be suitable for the most people followed by the soft.
If the distance between your slats os 4" or less and there is no flex in the slats (similar to the floor) then it’s also unlikely that your experience would be related to the support system under your mattress.
While you may not feel the difference initially … a thicker wool mattress protector or mattress pad can also affect the feel and response of the mattress and in some cases can interfere with the ability of the comfort layers to contour to the shape of your body which may have some effect on support/alignment and comfort/pressure relief. It may also have a bigger effect on how you feel when you wake up in the morning than what you feel when you first go to sleep at night. A thicker protector or mattress pad may also extend the length of time that it takes for the mattress to break in. It may be worthwhile trying the mattress for a few nights without it to see if it makes any difference.
There are also some suggestions in post #2 here that may be helpful but I would make sure that you make a point of calling and talking with them because they have a great deal of experience in the industry and may be able to identify some changes and/or make some effective suggestions that will help you as well.
Of course the worst case is that nothing works out as well as you hope and then in 30 days you can just return the mattress for a refund but even if none of options you have work well for you if you spend some time testing the different options that you have available it can also contribute to your learning curve about the type of mattress that will end up having better odds of success.
Phoenix