Hi tdoyon,
There’s certainly nothing in your post that I would take any offense with
Unfortunately it’s not possible to “diagnose” mattress comfort issues on a forum with any certainty because you are the only one that can feel what you feel on a mattress and there are too many unique unknowns, variables, and complexities involved including body type, sleeping positions, health conditions, and individual sensitivities and preferences that can affect how each person sleeps on a mattress in terms of “comfort”, firmness, and PPP (Posture and alignment, Pressure relief, and your Personal preferences) or how they may affect any “symptoms” you are experiencing and a mattress that would be “perfect” for one person or even a larger group of people may be unsuitable for someone else to sleep on.
Having said that … there is more about the most common symptoms that people may experience when they sleep on a mattress and the most likely (although not the only) reasons for them in post #2 here.
There is also 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” that may also be helpful.
These posts are the “tools” that can help with the analysis, detective work, or trial and error that may be necessary to help you learn your body’s language and “translate” what your body is trying to tell you so you can make the types of changes that have the best chance of reducing or eliminating any “symptoms” you are experiencing.
Any changes in your “symptoms” when you sleep on a new configuration for a few days compared to your previous “symptoms” on previous configurations are generally a much more reliable indication of the type of layering changes or the “direction” of any firmness changes that would have the best chance of success and would be much more reliable than trying to assess the effect of the changes based on body measurements rather than your actual sleeping experience.
Knowing how and “how much” your lower back symptoms changed between combination #2 and #3 would be helpful.
Resolving mattress issues with a component mattress generally involves a process of differential diagnosis using probabilities and some trial and error in combination with assessing how your actual symptoms change and get “better or worse” with each layer combination you try along with a more detailed conversation with the retailer or manufacturer you purchased from to help identify the “direction” or types of firmness changes or which combination of layers would be likely to have the best chance of success based on your actual experience with each combination.
My “best” suggestion would be to have a more detailed conversation with Spindle that provides them with as much detail as possible so that they can use their knowledge and experience based on the “averages” of other customers that have had similar experiences to yours to provide you with some guidance and their “best guess” about the types of changes that would have the best chance of success.
There are several variables that can affect how soft or firm a mattress feels besides just the firmness of an individual layer or layer combination (see post #4 here) and adding a firm layer on the bottom can also result in a mattress that “feels” softer because of the effect of the additional thickness (there is more about the effect of thickness in post #14 here).
Adding a firm layer on the bottom would also result in using one of your layers as a topper and a layering combination that uses a layer as a topper can also feel softer than having the same layer inside the mattress cover because a separate topper can “act” a little more independently and “feel” a little softer compared to having the same layer inside the cover (see posts #3 and #4 here and the first page of posts in this topic).
While the most common cause of lower back pain is a mattress that in one way or another is too soft (either deeper layers that are too soft or upper layers that are too thick and/or soft) … it’s certainly possible that a soft layer that either replaced one of the medium layers in your mattress or that was added as a separate topper would provide additional secondary support and pressure relief that may also resolve the issues you are having but the only way to know for sure would be based on your actual sleeping experience.
Phoenix