Hi kate,
The most common cause for lower back pain is a mattress that is too soft (either has comfort layers that that have too much thickness/softness or a support core that is too soft and is “allowing” your hips/pelvis to sink down too deeply into the mattress) but this certainly isn’t the only cause for lower back pain and a mattress that is too firm also wouldn’t provide good support for the more recessed parts of your body (such as the waist or the small of the back) and can cause lower back pain and discomfort as well. A more extreme example of this would be sleeping on the floor which could certainly cause lower back pain for most people (and pressure points as well). Unfortunately you are the only one that can feel what you feel on a mattress and there is no way for me to know for certain which one would be the reason in your case outside of your own personal experience and comparisons with different combinations.
I would only try this after you have spent “enough” time on the polyfoam in combination with the 30 ILD topper.
The “recommendation” you are referring to is only a very general suggestion and is for a “medium firm” mattress … not necessarily medium firm top layers. Most medium firm mattresses have softer layers on top of a firmer support core and the firmness rating is the result of all the materials and components “as a whole” … not a specific layer.
I would keep in mind that there are also no “standard” definitions or consensus of opinions for firmness ratings and different manufacturers can rate their mattresses very differently than others so a mattress that one manufacturer rates as being a specific firmness could be rated very differently by another manufacturer. Different people can also have very different perceptions of firmness and softness compared to others as well and a mattress that feels firm for one person can feel like “medium” for someone else or even “soft” for someone else (or vice versa) depending on their body type, sleeping style, physiology, their frame of reference based on what they are used to, and their individual sensitivity and perceptions. There are also different types of firmness and softness that different people may be sensitive to that can affect how they “rate” a mattress as well (see post #15 here) so different people can also have very different opinions on how two mattresses compare in terms of firmness and and some people may rate one mattress as being firmer than another and someone else may rate them the other way around. This is all relative and very subjective and is as much an art as a science.
“Support” is often misunderstood and many people believe incorrectly that “firmer is better” or “more supportive” when the real goal of a “supportive” mattress is to keep the spine and joints in good alignment and this requires the type of contouring support that allows some parts of the body to sink in more (softer) and some parts of the body to sink in less (firmer) and this will vary on an individual basis. 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/alignment” and “comfort/pressure relief” and “feel” and how they interact together.
Phoenix