Hi sagefemme,
The best source of guidance would normally be the manufacturer themselves who will have more experience with their mattresses and adjusting or exchanging layers (for the many different types of people with different body types, sleeping positions, and preferences) than anyone on the forum (including me). In many cases if you ask for specific suggestions from a large group of people with very different body types, sleeping positions, sensitivities, and preferences than you then you will get so many conflicting suggestions that have worked well for them but may not work as well for you that it can end up becoming more confusing than helpful.
I (and SleepEZ as well) would agree that if your ideal configuration ends up with a soft layer on the bottom (because there is nowhere else to put it) then it would probably be better to exchange it for a firmer layer.
I would be very cautious with this because it will depend on what “type” of firmness he means (see post #15 here). For most people an extra firm comfort layer would end up being firmer than they would end up preferring. His sense of “firmness/softness” may be coming from the deeper layers.
In very general terms … the top layer is mostly about pressure relief, the deeper layers are more about support, and the middle layers can contribute to both. Moving firmer layers closer to the top and softer layers deeper in the mattress will generally improve support. Moving softer layers up and moving firmer layers down will generally improve pressure relief. Changing the “feel” is just a preference issue and would be based on your own experience rather than on any “symptoms”. A mattress protector, mattress pad, or other thin layers on top of your mattress along with your sheets and bedding can also change the “feel” of the mattress. It’s important to know which of these you are trying to change and how you are trying to change them (support/alignment, comfort/pressure relief, or feel and personal preference issues) because each one can require a different type of change.
Post #2 here and the posts it links to have much more information about the different types of “symptoms” that people may experience on a mattress and some suggestions about the possible underlying causes for them and the types of changes that can be effective but this can become quite complex for those who are not familiar with mattress design theory and the effects of changing or rearranging different layers and it’s important to make the “right changes” to the “right layers” to get to the combination that is best for you and this would generally take more specific information than you have provided.
You will usually be better off talking with SleepEz where you can provide much more detailed and specific information that is relevant to you and have a more detailed and “interactive” conversation in “real time” about the specifics of what you are trying to change and the actual symptoms you are trying to alleviate and whether any issues you have are related to comfort/pressure relief, support/alignment, or just the “feel” of the mattress.
With their layer exchanges they will also ship you the new layer first so you can use it to try different combinations before you decide on which layer to send back.
Phoenix