Hi rdr93,
As you mentioned … the layering of these two iComfort mattress are very different and it’s not really possible to “match” one mattress to another without a lot more information about the details of every layer. There is more about “matching” one mattress to another and the difficulties of doing this in post #2 here. There is also a brief outline of the three ways to duplicate another mattress in post #2 here.
None of these would be possible in your case because Serta doesn’t provide enough information about the specific materials they use and you would need to change the layers inside the mattress not just add a layer on top to “match” the two.
So the most effective method would be to try to match your current mattress to a “target” that is more about the specific qualities you are looking for than another mattress. for example … the 2" top layer of memory foam in the Savant may feel completely different in another layering combination than it does on the Savant because every layer in a mattress is affected by the layers above and below them in terms of how they feel and perform.
The density of the polyfoam in the upper layers of the icomfort line is 1.35 lbs (low quality) but density has little to do with how a material feels. Every density of polyfoam can come in firmer and softer versions and density is more about the durability of a material and how long it will keep it’s original specs and properties. For example the density of the 2" polyfoam layer in the Genius is low but it is very firm while the polyfoam in the Savant is soft. This firmness won’t last however and I have already heard from retailers that have had complaints about the Genius softening from its original specs in a relatively short time (under a year) because of the density of the polyfoam they use.
So I would tend to use a 2" memory foam topper in a similar density (the cool comfort is 5 lb memory foam) and feel (you will have to trust the descriptions of the memory foam you are looking at or conversations with the retailer that sells them in terms of how breathable it is and how it feels because there is no accurate way to compare the many different memory foams to each other except in very general terms). While this probably won’t “match” the feel of the Savant … it will probably move you closer to the performance and feel you are looking for (more pressure relief and a softer feel).
Of course you could also add latex which is available in many different firmness levels (from very soft to very firm) but this will have a very different feel than a memory foam topper and it would depend on whether you preferred a fast response topper or a slower response topper. While the pressure relief can be similar between the two materials, the “feel” will be very different. Latex will be more breathable and cooler than memory foam however especially if you also have a more breathable protector and sheets over it (the sleeping temperature of a mattress depends on how all the materials in the upper part of the mattress and the bedding layers perform together more than on the properties of any particular layer).
What feels “soft” for you will also depend on your height and weight and on your sleeping positions and preferences (every person has a different definition of soft or firm) but in general … 2" of 4-5 lb “cool” memory foam or 2" of softer latex (with an ILD that you would feel as soft but probably in the 19 - 24 ILD range) would probably be close to what you are looking for.
Some of the better sources of both memory foam and latex listed in post #4 here.
Phoenix