Hi sheckylovejoy,
Perceptions of softness and firmness are subjective and relative to each person but I think in general that most people would describe the “feel” as being in the range you mentioned. All foams will soften over time depending on many factors but the primary factor in durability is foam density so a 4 lb memory foam would tend to soften more slowly than lower density memory foam and faster than higher density memory foam. There are many other factors involved though and there is more about durability in post #4 here and some of the other posts it links to.
Memory foam has many different properties that depend on the chemicals that are used to make it which can affect many of its properties much more than the density (see post #9 here). Density is directly connected to durability and to some degree to the response time and ventilation of memory foam (lower densities tend to respond more quickly and tend to be more breathable) but firmness isn’t linked to density and any density of memory foam (like polyfoam) can be made firmer or softer independent of density. The subjective feeling of firmness and softness with memory foam also depends on its temperature sensitivity and on how long it takes to soften with heat. Some memory foams may start off firmer initially and then as they warm up become softer and the “feeling” of softness is also based on the speed of movement (just like with honey if you slap it it will be very firm but if you press into it slowly it will feel soft). The Tempurpedic 7 lb memory foam for example is generally considered to be softer overall than their 5.3 lb memory foams (although they also have different versions of their 5.3 lb memory foam that are either softer or firmer). Higher density memory foams also tend to be more conforming and will also tend to sink in less overall (they can become firmer as you sink in more deeply than lower density memory foams). In other words … it’s usually best to avoid generalizations about memory foam based on density outside of knowing that higher densities are generally more durable (up till about 6 lbs or so when the difference levels off) and will also tend to warm up and soften more slowly and have a little slower response.
When you are matching one mattress to another it can match based on a general subjective “feel” or softness, based on the quality and density of the materials, or based on design (the thickness of the layers and how they are arranged) and in some cases all three. You can read more about matching one mattress to another in post #2 here. The Dreamfoam 13" gel memory foam mattress approximates the Cloud Luxe in terms of feel (it’s slightly firmer) and has similarities in design (It has 3" of 4 lb gel memory foam over 2" of 5 lb memory foam over an 8" 1.5 lb polyfoam base layer and the Cloud Luxe has 2.75" of 4 lb memory foam over 2" of 7 lb memory foam over two 4" 1.5 lb polyfoam base layers) but it doesn’t match in terms of the memory foam densities (although again this has little to do with the firmness or softness of a memory foam). They provide the comparison to the Tempurpedic mattress so that their customers have a general approximation as to how the mattress “feels” in terms of softness based on most people’s perceptions of softness.
A good indication would be to carefully and objectively test the Tempurpedic Cloud Luxe (for at least 15 minutes and preferably longer to give the memory foam time to warm up) and if the softness of the comfort layers (pressure relief) and the firmness of the base layers (support) are suitable for you and you would be OK with what people would generally consider to be similar (a little firmer) mattress then the odds are good that it would work well for you. Both of these mattresses would generally be considered to be “soft” (the Cloud Luxe is generally considered to be Tempurpedics softest mattress) at least in terms of the comfort layers which is generally what most people “feel” the most.
You can see some suggestions for mattress protectors in post #89 here. The membrane type that are waterproof (or at least fairly waterproof) are generally quite similar and my sense is that most of the complaints on Amazon (which you will see for all of them to different degrees) are based on care instructions that weren’t followed although there may also have been some legitimately defective ones as well. If your cat claws through it then it would possibly allow any accidents to leak through but these are the only ones that are “mostly waterproof” outside of the solid vinyl type that don’t breath at all (will sleep warmer) and have that “crinkly” type of feel to them which most people don’t like at all. So the membrane type (and they would all tend to be very similar) in practical terms are the ones that most people who want something waterproof would generally choose.
The wool protectors are more “water resistant” but not “waterproof” although the NaturaProtect Deluxe that is linked there has both wool and the membrane so it may be more protective then either one by itself and the wool may protect the membrane from cat claws but it’s also more costly and would have a little more effect on the memory foam below it than a thinner protector without the wool. I own one of these and it has held up well for about 3 years so far and through quite a few washings.
Phoenix