Hi DesertDuck,
Most people wouldn’t feel any difference between two support systems that are both have a rigid non flexing support surface so it’s surprising to me that you would feel a difference between them. It’s more likely that the difference was because of changing the memory foam layer but either way it’s good to hear that your back issues have gone away.
A mattress with a latex support core will generally do best with a rigid non flexing support system underneath it and layers that are closer to the sleeping surface will have a much more noticeable effect than layers that are deeper in your sleeping system (and some people may not feel much difference at all by adding a softer layer under your mattress) so if you are looking to soften your mattress I would definitely go in the direction of adding a soft topper.
If the topper you add is also memory foam then it will still have the memory foam “feel” although some people prefer the “feel” of having a more resilient material on top of memory foam as well.
In very general terms … the properties and firmness of materials and components that are closer to the top surface of a sleeping system will tend to have a bigger effect on the overall “feel” and firmness of a mattress than materials that are deeper in the sleeping system, thicker layers or toppers will contribute more of their feel and firmness to the overall sleeping system than thinner layers, and a thinner layer would “allow” more of the feel and properties of the layer(s) underneath it to “come through” than a thicker layer.
It may be worth reversing the order of your two latex layers for a few days to see if that makes a difference in your wife’s sciatic pain and the type of difference it makes (better or worse) because it’s possible that her pain could be coming from support layers that are too soft so you could test the effect of firming up the support system before adding a softer topper.
I would also keep in mind that there will be a break in and adjustment period for any new mattress or topper as the topper loses any of it’s “false firmness” and your body gets used to a sleeping surface that is different from what it is used to (see post #3 here). This could typically be a few weeks but it can be shorter or longer depending on the specifics of the person and the mattress (higher density materials can take longer) and it can be surprising to some people how much their sleeping experience can change over the course of the first few weeks. It may also be helpful to walk evenly and carefully across the surface which can help speed up the initial softening or break in period.
If the only issue with a mattress is that it is too firm and there are no soft spots or sagging in the mattress then a good quality topper can certainly be an effective way to add some additional softness, “comfort” and pressure relief to your sleeping system but the only way to know for certain whether a specific mattress/topper combination is a good “match” for you in terms of comfort, firmness, and PPP is based on your own careful testing or personal experience on the combination. If you can’t test the combination in person then there will always be always some risk and uncertainty involved in adding a topper because the specifics of the mattress itself along with your own body type, sleeping position, and preferences can affect which specific topper would be a suitable choice on any specific mattress.
There is more information about choosing a topper and a link to the better online sources I’m aware of in post #2 here and the topper guidelines it links to which along with a conversation with a reliable and knowledgeable supplier (that can provide you with good information about how their toppers compare to each other or to other toppers they are familiar with that are available on the market) can help you use your sleeping experience as a reference point and guideline to help you choose the type, thickness, and firmness for a topper that has the least possible risk and the best chance for success. A good exchange/return policy can also reduce the risk of an online topper purchase so I would make sure you are comfortable with the options you have available after a purchase just in case the topper you choose doesn’t turn out as well as you hoped for.
Phoenix