Hi lotus65,
I’m sorry to hear that you are experiencing back pain on your mattress!
Based on quickly reading some of your previous posts and some of your previous experiences … I would keep in mind that there may be no perfect solution because when you have pre-existing back issues a mattress may work well for you over a period of months and then no longer work as well if something changes and your back becomes more sensitive (see a very insightful post from another member in post #1 here).
If you experience back pain when you wake up in the morning that goes away shortly after you get up then it is most often the result of a mattress that is too soft but if the back pain tends to continue over the course of the day then it’s most likely the result of “non mattress” issues that may need to be resolved.
It’s also possible that if you choose a mattress that is “on the edge” of being too soft for your body type and sleeping style when it is new then even a small amount of foam softening that would be in a normal range could be enough to put you over the edge and a mattress that worked well for you at first may no longer be as suitable to keep you in good alignment over the course of the night even though the mattress itself may not have any unusual issues (see post #2 here).
“Fixing” a mattress that is too soft or that has developed some soft spots or virtual or visible impressions is very difficult because the most effective solution is usually removing and replacing the layers that are (or have become) too soft but there are some suggestions in post #4 here that may be helpful at least partially or temporarily (or in some cases can be a more long term solution).
If your lower back pain lessens with the plywood (or by putting the mattress on the floor) but the mattress becomes “uncomfortable” then you may have resolved one issue but traded it for another one. This would actually be good news because it would indicate that your foundation was sagging and was likely contributing to the back pain (the mattress will follow any sag in the support system) but you may now need a topper to add the additional comfort or pressure relief that you need on your mattress. It would depend on what you mean by “uncomfortable”.
If you are in good alignment with the plywood and decide that all you need is a topper then post #2 here and the topper guidelines it links to can help you use your sleeping experience as a reference point and guideline to choose the type, thickness, and firmness for a topper that has the best chance of success.
There is more information about the more common “symptoms” that people tend to experience on a mattress and the more common reasons behind them (to the degree it is connected to the mattress) in post #2 here. The odds are higher that the symptoms you are describing would come from a mattress that is too soft rather than too firm but this may not always be the case because each person can be very different from the next and the same symptoms can come from several different causes.
Pillows that aren’t a good match for a particular person are usually connected to upper body issues so it’s less likely that a pillow is the cause but anything is possible. The best way to find out would be by a process of elimination and by using a lower profile pillow to see how it affects your lower back issues or if they are resolved.
It may take some trial and error to decide on the types of changes that may be most helpful but the most effective approach is to first eliminate the “non mattress” possibilities (foundation, any mattress pads or mattress protectors you are using, pillow issues etc) and to carefully decide on the types of changes that would have the highest odds of success and making small incremental changes to see how your symptoms change and the “direction” of the change (whether they made things a little better or worse).
I would always keep in mind that the design of a mattress and how well it matches your unique needs and preferences is always much more important than the types of materials and components that are inside it. There are hundreds of mattress variations in each mattress category and a latex mattress (or any type of mattress or materials) that is a “perfect” match for one person may be completely unsuitable for someone else to sleep on.
Phoenix