Hi cbb,
There are several “specs” that work together that will affect how soft or firm any single foam material will feel (see post #4 here) and some of these may not be available to you so in some cases a lower density foam with a lower compression modulus and higher ILD can feel softer than a higher density foam with a lower ILD. There are many different “versions” of each type of each type of foam or other components in a mattress and every layer and component in a mattress (including the cover) can affect the feel and performance of all the other layers as well (see post #2 here).
So what this means in “real life” is that trying to predict how a mattress will feel and perform using only one of many “specs” or variables that are involved can be somewhat risky or misleading and even the most experienced mattress designers that are familiar with and have a great deal of experience with all the many variables and specs that can affect the feel and performance of a mattress can often be surprised at what a mattress was “supposed” to feel like based on specs and what it “actually” feels like in real life so in practical terms the only reliable way to know how any specific combination of materials and specs will feel like for you would be based on your own actual testing or personal experience. Specs can be a very rough guideline that can help you “find the range” but there are far too many unknowns and variables to use them to find the “best” combination or materials or components that would be suitable for any specific person with any degree of certainty.
Phoenix