Hi lilac2,
There is no “standard” definition or consensus of opinion for firmness ratings and different manufacturers can rate their mattresses very differently than others so a mattress that one manufacturer rates as being medium firm" could be rated very differently by another manufacturer. Different people can also have very different perceptions of firmness and softness compared to others as well and a mattress that is firm for one can feel like “medium” for someone else or even “soft” for someone else depending on their body type, sleeping style, physiology, their frame of reference based on what they are used to, and their individual sensitivity and perceptions. This is all relative and is as much an art as a science. There are also different types of firmness and softness that different people may be sensitive to that can affect how they “rate” a mattress as well (see post #15 here). In other words you will always need to test a mattress for you to assess how firm or soft it feels to you (hopefully using the testing guidelines in the tutorial post) regardless of whether it feels firm or soft to someone else or how anyone else may “rate” it.
The first “rule” of mattresses is that you are the only one that can feel what you feel on a mattress and your own testing or personal experience will always be the only way to know for certain whether any mattress is a good “match” for you in terms of PPP. A manufacturer may suggest a particular mattress based on the information you provide them but you will always be the only one who can assess whether any mattress they recommend works well for you in “real life”.
I would also suggest talking to Magic Sleeper and they may be able to make some adjustments to the design of your mattress that will make it more suitable for you.
As I mentioned … they have much more experience with their mattresses than any of their customers so I understand that while you may also have your own thoughts about mattress design and theory … in most cases the experience of a manufacturer that actually makes a mattress and that has the feedback of thousands of customers over the years will generally be your best source of guidance.
The only reason they would have suggested what they did is based on their own belief about what would work best for you since a box spring would have increased the price of the set (and their profit when you purchased it) but if you believe that a box spring would be a better choice with your mattress then they certainly have them available so you could test the combination in your own real life experience. It’s always possible that you could be the exception to the experience of most of their customers and what works best for “most” people isn’t always the best choice for “all” people because each person is unique.
Phoenix