Hi Ivehadit,
There is really no easy or specific answer to this question because it depends on the design of the mattress and which layers you are talking about as well as how each person interacts with each design based on the differences in their body type, sleeping style, and individual sensitivities and even physiological differences. As you can read in post #4 here and in post #5 here … the goal of a mattress is to provide you with the PPP (Posture and alignment, Pressure relief, and your Personal preferences) that is closest to your personal ideal. This will take a combination of firmness and softness in different layers that work well together with your body type and sleeping style.
I will say though that at your weight much firmer than the norm would be a better choice because heavier weights will perceive firmer layers as being softer than lighter people.
Your experience of testing is also not unusual in terms of the “residual effect” of testing some mattresses and it’s usually a good idea to wait a bit before testing more when this happens or when you lose the ability to differentiate between mattresses because of “testing fatigue”.
You are in good hands and I would take your time. I would also remember that a little firmer is usually a little safer choice than a little softer because you can always soften up a mattress that is too firm over time but you can’t improve the support of a mattress that is too soft by adding layers on top and you would need to remove or replace layers in this case.
In general terms … most people today are used to mattresses that are probably too soft for their needs although mattresses that are too firm (firm enough that they produce symptoms of pressure relief or alignment issues when you sleep on them) are also not good choices. It’s always the balance between the conflicting requirements of pressure relief and alignment which produces the “ideal” mattress for each person.
Phoenix