Hi RJ29,
That’s probably too broad a question for anyone to really answer (“other combination beds” would include hundreds to thousands of mattresses that would be very different from each other) but there is more information in post #9 here about the different ways that one mattress can “match” or “approximate” another one. Every layer and component in a mattress (including the cover) will affect the feel and performance of every other layer and component and the mattress “as a whole” and unless you are able to find another mattress that uses exactly the same materials, components, and design (which would be very unlikely) then there really isn’t a way to match one mattress to another one based on the specifications of the mattress.
Mattress manufacturers generally try to differentiate their mattress from the mattresses made by other manufacturers and don’t normally try to “match” another mattress that is made by a different manufacturer so it’s also unlikely that you will find another mattress that is specifically designed to “match” or “approximate” one of the mattresses you mentioned in terms so the only way to know how two mattresses compare for you in terms of how it “feels” or in terms of firmness or PPP would be based on your own careful testing or actual sleeping experience on both of them.
There are also no “standard” definitions or consensus of opinions for firmness ratings and different manufacturers can rate their mattresses very differently than others so a mattress that one manufacturer rates as being a specific firmness 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 feels firm for one person can feel like “medium” for someone else or even “soft” for someone else (or vice versa) 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. 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) so different people can also have very different opinions on how two mattresses compare in terms of firmness as well and some people may rate one mattress as being firmer than another and someone else may rate them the other way around. This is all relative and very subjective and is as much an art as a science.
There is also some good information about the most important parts of the “value” of a mattress purchase in post #13 here which can help you make more meaningful quality/value comparisons between mattresses in terms of suitability (how well you will sleep), durability (how long you will sleep well), and the overall value of a mattress compared to your other finalists (based on all the parts of your personal value equation that are most important to you).
Forum searches on MiComfort and on Alexander and on BestMattressEver (you can just click the links) will also bring up more comments and feedback about each of them.
Phoenix