Hi daveatronic,
Thanks a lot for your responses. As for the other information, I am about 5'9" 140-150 lbs (so fairly slim). I'm a back sleeper with no particular pain issues. My wife is 5'4", usually around 115-120 (also fairly slim), however she's pregnant at the moment. She is a side sleeper with a little intermittent lower back pain, but not a big issue.
It appears that things are working fairly well for you if there are no actual "symptoms" of concern so I'm not sure what specific property of the mattress you are targeting in making any changes.
If the mattress is working well for you (you have no alignment or pressure symptoms) and for your wife as well except that she has occasional back issues which could be from the pregnancy and are not of particular concern either (although a
pregnancy pillow
could be helpful if the back issue is from the pregnancy) then are you just trying to change the "feel"?.
I noticed that the standard Brooklyn Bedding total latex bed bottom 6" layer is typically 32 ILD so I am wondering whether that firmness is ideal -- unfortunately, it doesn't appear to be an option with Dixie.
All the layers of a mattress interact together and affect each other and there is no such thing as an "ideal" layering ... only a specific design that is most suitable for each individual person based on their body type, sleeping positions, sensitivities, and preferences. The goal of a mattress is always to keep the spine and joints in good neutral alignment in all your sleeping positions and to relieve pressure. Everything else is a preference. With the softer support cores you referenced ... the comfort layers are also softer and the comfort layers are a bigger part of how a mattress "feels" when you lie on it than the support cores.
The thickness and softness of the comfort layers are mostly responsible for "allowing" the pressure points (shoulders and hips mostly) to sink in enough to distribute the body weight and relieve pressure. They are also responsible for "filling in the gaps" of the body profile (what I call secondary support). The deeper layers are mostly responsible for the primary support of the mattress that "stops" the heavier pelvis from sinking down to far which is the most common cause of lower back issues. You could check to make sure that the gaps (in the waist for side sleepers and the small of the back for back sleepers) are being well supported by trying to slide your hand underneath the body. It should be fairly difficult to do this and there should be some good resistance if the foam under these areas are being compressed.
When you are a lighter weight then both the comfort layers and the support layers can be softer because there is less weight to support. Lighter body weights will also sink in less so thickness and firmness of the comfort layers becomes more important in terms of the "feel" of the mattress. With heavier weights the firmness of the support cores will come into play more because you will sink in more deeply into the mattress and "feel" more of the middle and deeper sections of the mattress.
So I'm not clear whether you are you trying to change support/alignment, pressure relief, or whether it's more of a preference of "feel" (which is the least important of the three).
How long have you slept on the mattress? It could also be that you are still adjusting to the "feel" of a new sleeping surface that is different from what you are used to.
It would also be helpful to know what type of mattress protector you are using as this can also affect the "feel" of the mattress and affect how well the foam on top compresses and contours to your body profile as well.
While the "feeling" of firmness and softness is relative to each person and is subjective ... it's possible that a softer top layer may also be helpful since this is the biggest part of what you "feel" ... especially when you are lighter. With your lighter weights both a 40 ILD and a 36 ILD would generally be firm enough to provide good support and "stop" the pelvis from sinking down too far although a softer support core would also have a secondary effect on the "feel" of the mattress as well (secondary meaning less noticeable for most people than a softer comfort layer).
Phoenix