Hi cownox,
It’s unlikely that you would notice any difference in the different design. As technology improves and changes over time … it’s very common that a manufacturer will switch suppliers or make other changes to a mattress and most good manufacturers are always looking for and testing new and better sources of materials that can offer better performance or value to their customers.
I don’t think that your sheets or foundation are an issue although a mattress pad as you discovered can certainly affect how a mattress feels and performs … for better or for worse.
When you are looking to make adjustments to your mattress … what it “feels like” is very subjective and not that reliable in terms of knowing what if anything needs to change and I would tend to focus on more specific symptoms (such as specific pressure points or discomfort or pain in your back).
The most important part of fine tuning a mattress is a phone conversation with the manufacturer who has more knowledge about their mattresses than anyone and is in the best position to make suggestions for your specific circumstances based on a more detailed conversation.
In some cases … split layering can also be very helpful when a couple has very different needs and preferences. Stomach sleepers tend to need firmer and/or thinner comfort layers than side sleepers to reduce the risk of sleeping in a swayback position.
I can only speak to the quality or value of a product and the products that BB supplies are certainly good quality/value. There are too many variables and unknowns in choosing a mattress and/or topper based on “theory at a distance” and only your own experience can really know how it will end up feeling and performing in combination with your mattress. The only way to know how any combination will feel and perform is personal experience. If this isn’t possible … then the guidance of a manufacturer or retailer based on “averages” (that may or may not apply to your specific circumstances, body type, and sleeping style) is the best way to make your choices. The benefit of using a topper is that it can add softness under the hips if you are feeling pressure points there. The thinnest topper that you believe will do the job is usually best to reduce the risk of replacing a pressure issue with an alignment issue. Once again … more detailed phone conversations with the manufacturer of your mattress is the best source of advice when it comes to your comfort and support. They have more experience with their mattresses than anyone. There are also some topper guidelines in post #2 here and the posts it links to that may be helpful.
If you are looking at a new mattress … my focus would be on a single variable which is getting the mattress right without a topper a all. If you make the wrong comfort choice … then you can either make changes to the mattress if there are options to re-arrange or exchange a layer or you can choose a topper for fine tuning based on your actual experience but I certainly wouldn’t set out buy both a topper and a mattress. Once you have slept on the mattress by itself … then you can use the topper guidelines in combination with more detailed conversations with a knowledgeable retailer or manufacturer to make the best possible choices.