Hi Thatg,
The density of memory foam has little to do with its softness and more to do with it’s point elasticity (contouring ability), compression modulus (how quickly it gets firmer as you compress it more deeply), and durability (up to about 6 lbs or so after which it makes little difference) so it would really depend on how the specific properties of a higher density memory foam “fits” with the rest of the materials in the mattress as a whole and with your specific needs and preferences. I don’t think that any of the memory foams on the market are inherently “too dense” no but some memory foams (regardless of density) may have some properties that in combination with the other materials and components in a mattress may mean a mattress that uses them may not be suitable for you in terms of PPP. Density is only loosely related to the properties of memory foam and there is a wide variance between the properties of different memory foams even if they are the same density (see post #9 here and post #8 here).
The word “support” can also be misleading when it’s used in the context of memory foam because memory foam is only used as a comfort or transition layer and not as a primary support material (it’s too soft to be used as a support layer) so describing memory foam as “supportive” is only relative to other memory foams in general and not to materials that are used in a support layer (such as an innerspring, latex, or the polyfoam that is usually used in a memory foam mattress).
The goal of a mattress is to provide different levels of support under different parts of the body because how much you sink into a mattress isn’t as important as how evenly you sink into a mattress and your spinal alignment in all your sleeping positions. 7 lb memory foams can be softer than 5 lb memory foam (the Tempurpedic 7 lb memory foam is an example of this) but it also has a higher compression modulus (gets firmer faster than lower density memory foams) so it can be both softer and more “supportive” (gets firmer faster as you sink into it). This can help provide the comfort/pressure relief that is needed but at the same time provide better support/alignment for higher weights. There is more about primary support, secondary support, and pressure relief and the relationship between them in post #4 here.
When you are a stomach sleeper then thinner comfort layers that keep you closer to the firmer support layers of a mattress can be important so that you have some initial softness and pressure relief but your heavier pelvis doesn’t sink too deeply into the mattress relative to the other parts of your body which can lead to sleeping out of alignment in a swayback position which can lead to lower back discomfort and pain.
There is really no “firm” memory foam (the firmness of memory foam is only relative to other memory foams) and when you are a stomach sleeper then the thickness of the memory foam comfort layers and how much of the firmness of the support layers “come through” may be more important than the firmness of the memory foam itself.
Most stomach sleepers also spend a significant amount of time in other sleeping positions and while I wasn’t on the call and I don’t know the specific reasons they may have made the suggestions they did … their concern may have been that the 3" of 5.3 lb memory foam in the Select HD may have been too thin or firm or not conforming enough to keep you in good alignment and not have enough softness and contouring to fill in the gaps in your sleeping profile and be evenly supportive enough in your other sleeping positions even though it would certainly be durable enough (you are in a weight range where 4 lb memory foam or higher would likely be suitable in the right thickness and design).
Phoenix