Hi Suffolk,
This is really a very difficult question that has no specific answers on an individual level because it can involve some very complex and interacting factors including the specific layers or specific area of the mattress (under the hips, under the shoulders etc) that may be “too soft” or “too firm” rather than the mattress as a whole.
The goal of all theory and testing is to try and predict which type of mattress may provide the best possible PPP (Posture and alignment, Pressure relief, and Personal preferences) in actual experience over the long term and this can vary widely even between different individuals of the same height and weight and sleeping styles. There is an infinite number of variables and variations between people including specific physiology and sensitivities and variations of sleeping positions that can make a significant difference and “theory” is often just a starting point that needs to be confirmed with personal experience and in some cases trial and error (based on either local testing or actual experience).
Stomach sleeping is the most risky sleeping position and in “theory” … while the mattress you chose would be a good average choice for someone of your height and weight that was a side or back sleeper the layers may be a little too thick and soft for a stomach sleeper which generally requires thinner and/or firmer comfort layer so you are closer to the firmer support system of the mattress and which can “stop” the heavier parts of your body from sinking down too far and hyper extending the lumbar. All of this would also depend on your weight distribution and how evenly your specific body type and sleeping position sinks into the mattress.
Given your specific sleeping style (stomach sleeper), your symptoms, and the construction of your mattress … I would say the odds are higher that the mattress is too soft than too firm … particularly under the pelvis.
If a comfort layer is too thick and/or soft … then “theory” says that adding more soft layers on top of the mattress is not normally an effective solution (it can allow the pelvis to sink in even deeper relative to the other areas of the body) although there are some circumstances where it can make some difference at least on a temporary basis. In most cases where one part of your body is sinking down too far relative to the others then replacing either the support layers with a firmer option, exchanging the comfort layers with a thinner or firmer version, or adding thickness under the part of the body that is sinking down too far (usually under the mattress) have the best chance of “fixing” a mattress that is too soft under certain areas of the body. This is why it’s generally much easier to “fix” a mattress that is too firm by adding additional softness on top than it is to correct a mattress that is too soft which generally requires removing or replacing layers for something thinner or firmer. Post #4 here has more about some of the potential options for a mattress that is too soft under some areas of the body.
It’s also important to remember that “comfort/pressure relief” is generally what you experience when you first lie on a mattress but “support/alignment” is what you experience when you wake up in the morning and can be more difficult to test for because it requires a combination of visual cues and more attention to the more subtle cues of your body that can give you “hints” of some of the symptoms can take time to become more obvious.
While there are no specs that indicate the firmness of this mattress … this points to the likelihood that the box spring was probably sagging in the middle after a year of use. This could also be in combination with the mattress softening under the pelvis more than it softened under the lighter parts of the body. One or both of these would contribute to your pelvis sinking down too far and sleeping out of alignment. Replacing the box spring with something that didn’t compress as much in the middle would normally help although it may only be a partial or temporary solution solution because part of the issue after a year could also be the softening of some of the foam layers under the hips and it’s possible that the new foundation would not help as much over the longer term if it is only part of the problem.
“Theory” also says that this mattress would also be a “risky” choice for a stomach sleeper because the comfort layers are fairly thick/soft and the support layers are also on the softer side. “Liking” a mattress will not usually reflect your long term experience when you sleep on it and a mattress may be very “comfortable” in a showroom but not provide good PPP when you actually sleep on it. It’s always best to test as accurately, objectively, and specifically as possible for alignment because “liking” is usually much more subjective and may not reflect how well you are in alignment on the mattress. When a mattress has a risky construction especially (generally softer is riskier) I would pay particular attention to testing specifically for support because when you actually sleep on it over the course of the night or several nights then alignment issues become much more obvious.
In general and in “theory” … what you would be looking for when you experience low back pain is to “bring up” the pelvic area relative to the other lighter areas of your body (or in some cases to provide more secondary support that helps to support the more recessed gaps in the waist or under the lumbar curve but this doesn’t really apply to stomach sleepers). Beside making the comfort layers thinner or firmer the options that have the greatest odds of success would be to put something under the pelvis to see how this affects your alignment and “symptoms”. The easiest way to test this would be to put a thin pillow under your pelvis to “bring it up” to see if this makes a difference. If it does … then the best chance of improving things when changing the comfort layers aren’t possible would be to put something under the mattress that increased it’s thickness and height under the parts of the body that need more support to bring the spine into better alignment.
The general “rule” is that you can “fix” comfort from the top down but fixing alignment needs to be from the bottom up.
Phoenix