Hi amnj,
This is great, as the only way to know whether any specific mattress design or combination of layers and components is a good “match” for you in terms of comfort, firmness, and PPP with any certainty will be based on your own careful testing and/or your own personal experience when you sleep on it. It’s nice that you have the option of testing out these configurations in person.
Yes, you are correct, this is the most traditional design, what I term a more “progressive design”.
Certainly, it is quite common for people to do this, for example a design of, top to bottom, Medium, Plush, Firm in the configuration you’re considering. It generally provides a bit less of a “point elastic” surface comfort, but as long as the top layer isn’t too firm or thick so that it totally dominates the layer beneath it, it can provide a sleeping surface that is a bit less “mushy” and tends to feel a bit more “crisp”, which is what I think you are trying to achieve (or at least avoid the quicksand feeling). You can read more about this in post #33 here and the posts it links to.
If you’re curious, there is more about primary or “deep” support and secondary or “surface” support and their relationship to firmness and pressure relief and the “roles” of different layers in a mattress in post #2 here and in post #4 here that may also be helpful in clarifying the difference between “support” and “pressure relief” and “feel”. You always want to make sure you have good support and cater to overall alignment.
It is a less common way to soften up a mattress, but it can provide a bit of overall extra plushness to a mattress that has already been completed when the desire is for just a bit of extra softness with as little disturbance of the feel in the uppermost layers of a mattress. I remember this configuration was applied in the short-lived Pure Latex Bliss Prestige line in their St. Honore beds, but these were essentially already finished 11" mattresses with 2" of softer latex placed at the bottom of their configuration. In a 9" latex mattress using three 3" individual layers, using a plush lower 3" layer isn’t something that I would normally recommend and is not commonly pursued.
Even if your knowledge of physics was still fresh, trying to predict the feel of a mattress and completely describe the interactions of multiple layers, what I call “theory at a distance” is incredibly complex and even experts in mattress design with decades of experience are often surprised at the feel of their creations. There is as much art as there is science in creating a mattress. Having said that … you can see some general comments about the properties of an “ideal” mattress in post #4 here.
Let us know if you find a configuration that you like after your shopping visit.
Phoenix