Hi Lev,
You are a good example of how seemingly small differences can make a significant difference to some people.
The first thing I would suggest (if you haven’t done so already) is to give each change enough time for you to better evaluate it and monitor any “symptoms” or patterns rather than just a few days or so for each combination.
The second is that a “feeling” of softness is not indicative of the support of a material. Softness has many different versions … not just degrees of the same version … and softness used to describe the hand feel or surface softness of a mattress (which is what some people mean when they talk about softness) is very different from softness used to describe pressure relief (which is what other people mean when they describe softness) which in turn is very different from softness used to describe the support qualities of a mattress (which is what other people yet mean when they talk about softness) or even their perception of the “overall softness” of a mattress. Each of these are different “species” or types of softness and each type has many different degrees or variations.
Different materials than you are used to will often have a different “feel”. Before you “evaluate” whether a certain layering is too soft or too firm … it’s important to look at any specific symptoms that are being produced instead of what it 'feels like" when there are no specific symptoms involved. The feel of a certain material can take some getting used to but if it isn’t providing the pressure relief or the alignment that you need there will be specific symptoms of pain or discomfort in certain parts of the body involved. Because of its unusual qualities and response to pressure … latex can feel both softer and firmer at the same time than the materials that people are used to and it’s important to go by actual symptoms rather than just a 'feel" which may just be part of an adjustment period to a new and very different material. The last part of fine tuning is “feel”. Pressure relief and alignment are always the most important priorities.
I should also mention that how far you sink in to a mattress when you are sitting up and reading has very little to do with the support of a mattress which is designed for the weight distribution of a person when they are lying down. Latex in particular is so point elastic that you may sink in deeper when you are sitting on it and the weight is more concentrated (the material beside the point of compression has less effect or “drag” on the material that is compressing) and yet this same soft material is also very supportive and can provide good alignment when you are lying on it. Support (the ability to bear weight without bottoming out) is a means to achieve alignment when you are on a mattress in all your sleeping positions but neutral alignment is the goal and support in each area of the body is just the means to alignment … not the goal in itself.
Changing layer thickness is getting into some complex issues that may be beyond your ability to predict and I wouldn’t add unnecessary complications to the fine tuning process. If I was in your shoes … I would use the layering possibilities you have that is closest to your ideal (perhaps just a bit firmer) and then add a topper or mattress pad for the final bit of fine tuning. Starting to play with layer thickness introduces many more variables into the design and while it is a valid option … my sense is it would be more difficult and complex to fine tune than tweaking what you have now.
This generally indicates that the S/F/F is actually too firm and not too soft (which is why your arms may be falling asleep). When you have a softer layer on top … it will allow you to “go through” it more and you would be feeling more of the firm layer below it and the transition may be too much. Generally the solution to this would be to add a little more thickness to the surface (to isolate you a little more from the firmer transition layer) or to use a medium layer on top (which wouldn’t allow you to “go through” it as easily as the soft and reduce the effect of the firm layer. In other words … you are changing both the surface “feel” of the mattress which may “feel” softer or firmer to you based on the ILD of the top layer but you may also be changing the pressure relieving qualities of the mattress in a different direction. If you were to put a really soft layer on top for example … the surface or hand feel would be even softer but you could have more pressure relief issues because the firmness of the next layer down would come through more. Many things in mattress design and construction can be counter-intuitive.
Overall … it seems like the layer thickness is the issue with the soft on top and the general “fix” for this would be to replace the middle layer with a medium but this also seems to be too much in the other direction and you are in between. For the large majority of people either one or the other would be fine but you are more sensitive to smaller differences it seems. MMM may also be similar but again this would be a different “feel” even though it may be similar to another layering in terms of pressure relief and different once again in terms of alignment. This may also indicate than a topper of an inch or so (very small adjustments that are in between two options) may work well. Post #38 here may also be of interest which is about two toppers which can add some pressure relief with less risk to alignment than a solid piece of soft latex.
I would tend to work with the layers that you have and make sure you give each combination time for your body to adjust (again if you haven’t already). I would also focus more on actual symptoms than on “feel” which is something else completely. Once you have narrowed down the layering you currently have to the one that is closest to your ideal (in terms of pressure relief, alignment, and perhaps even feel as well) … then is the time to decide which of these needs any changing. Changing “surface feel” could involve different adjustments and options from changing pressure relief which in turn would be different from changing alignment. It’s important to first clearly identify … based on symptoms and descriptions that are as specific and objective as possible (sometimes the line between objective and subjective is very blurred) which of these you want to change.
Phoenix