Hi tskubala.
As I was reading through your post and the description of your current sleeping setup I realized that there may be some other factors you may need to consider.
As SleepEZ mentions, it is true that different latex manufacturers have different firmness ranges for the same stated density, but how firm the latex “feels” to each individual depends on their own perception, and to complicate things even more it is also related to the type of latex (Blended, NR, or SBR). Additionally, in the case of Dunlop (as SleepEZ mentioned), there are variances in density and ILD between the 3" top or bottom cuts sliced from a 6" core. This is why manufacturers will customize and pick the top or bottom by hand according to customer needs. Latex particles in Dunlop production settle more when they are poured into a mold and a 3" Dunlop layer that is cut from the bottom half of a 6" Dunlop core can be firmer than a 3" layer cut from the top half and the top would be softer than the bottom. With Talalay, however, density and ILDs are more consistent from top to bottom so it doesn’t matter as much which part of the core a thinner layer was cut from. All the layers cut from a single core will be rated the same even though they may not have exactly the same ILD.
To add to the above considerations, there may be (and most likely are) even more variables at play that you’d need to consider. To start off, it looks like your condition requires a very firm and even surface so I’d suggest a few experiments before attempting to exchange, replace layers, or take further action.
I’d first do a “ground up” assessment to make sure that there is nothing under the mattress that may be contributing to a “bend” of the latex into an existing “virtual” dip or sag within the mattress which generally wouldn’t be visible to the naked eye as most foams, rubber, and flexible materials have a strong tendency to rebound to a level that is not detectible when the weight is removed. These sags or dips can be sensed by the body only under the weight of the mattress and people sleeping on it. In your case these virtual impressions may be caused by several factors … for example, the sleep number air chamber fabric stretched or softened over time in certain areas or the “hard plastic slats” on which your sleep number sits started to lose their properties especially if there is no center support to the floor (for bed sizes larger than twins) or both. In this case, it’s less likely that any Xfirm layer added on top will be an effective solution because you can’t compensate for soft spots or dips because they will just “bend” into the softer areas underneath them and any effect they have will tend to be partial or temporary at best. If this is the case then there are some suggestions in post #4 here that may be helpful.
You can verify if this is the case by placing your Xfirm Dunlop layers on the floor on top of each other and sleeping on them for several nights to assess your alignment and need for more firmness and compare it with your previous setup. If you sleep better than either the foundation and/or sleep number may be"dipping" under the weight of the mattress, additional layers, and sleepers on top. and you’d need to determine which one, or if both are the culprits.
If the 3" layers placed on the floor still feel too soft you may want to further eliminate the top layer. This will also allow you to determine if your level of comfort or needs changed over time. I’d keep in mind that as we age, gain weight, lose weight, but especially with chronic health issues, skeletal and structural replacements, the muscles will typically tense or stretch to compensate for any minor or major changes. They will adjust and readjust in an attempt to allow the body to get restorative nightly rest. When the adjustment threshold is too high you’ll gradually feel the discomfort and end up waking up in pain.
Another factor to consider would be the thickness of the layers you used in the past vs the thickness of your new setup. Thickness and softness work together and the more thickness you add to the mattress the softer it will feel.
Hope this brings more precision to your perspective while looking for the best solution.
Phoenix