Hi brvogel,
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 and any quilting materials) will affect the feel and performance of every other layer and component and the mattress “as a whole” so unless you are able to find another mattress that uses exactly the same type of materials, components, cover and quilting, layer thicknesses, layer firmnesses, and overall design (which would be fairly unlikely) then there really isn’t a reliable way to match one mattress to another one in terms of “comfort”, firmness, and PPP (Posture and alignment, Pressure relief, and your Personal preferences) based on the specifications of the mattresses (even assuming that you can find out all the specifications you would need for both mattresses you are comparing in the first place).
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 unless a manufacturer specifically says in their description of a mattress that one of their mattresses in the same general category is specifically designed to “match” or “approximate” another one in terms of firmness or “feel” and PPP and/or they are very familiar with both mattresses and can provide reliable guidance about how they compare based on the “averages” of a larger group of people that have compared them (different people may have very different opinions about how two mattresses compare) … the only reliable way to know for certain how two mattresses would compare for you in terms of how they “feel” or in terms of firmness or PPP (regardless of anyone else’s opinions of how they compare which may be different from your own) 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 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.
In other words … if two mattresses have different designs and materials then your own careful testing or personal experience is the only reliable way to compare two mattresses in terms of “comfort”, firmness, and PPP.
Comparing two mattresses in terms of durability is much more simple and more objective once you know the specifications of all the materials and layers in two mattresses you are comparing (see this article) because making durability comparisons is just a matter of comparing the quality and durability of the materials and components inside it and making sure there are no lower quality materials or weak links in the mattress that would be a reason for concern (see the quality/durability guidelines here). Again though I would always keep in mind that the quality and durability of the materials has little to nothing to do with how a mattress will feel or compare to any other mattress in terms of comfort, firmness, or PPP.
There is also more about the 3 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 suitability, durability, and all the other parts of your personal value equation that are most important to you (including the price of course and the options you have available after a purchase if your choice doesn’t turn out as well as you hoped for).
The better options or possibilities I’m aware of in and around the Palo Alto area (subject to making sure that any mattress you are considering meets your specific criteria and the quality/value guidelines I linked earlier in this reply) are listed in post #2 here.
I’m not sure you are clear between the difference between a box spring and a foundation. A box spring has springs inside it that flex under the mattress and there are very few one sided mattresses today that use them as a support system and in many cases they will invalidate a mattress warranty. Most mattresses in the industry today need a steel or wooden bedframe with a foundation that has minimal to no flex (vs a box spring that flexes) or a platform bed which also has little to no flex under the mattress. There are many in the industry that mix up the terminology between box springs and foundations even though they are very different products.
Whether your platform bedframe would be a suitable support surface for any particular mattress would depend on the specifics of the mattress and the design of the bedframe. In most cases the manufacturer of a mattress will be a reliable source of information about whether any specific foundation or a platform bed is suitable for the mattress you are purchasing but there is also more information about the different type of support systems (bedframes and foundations or platform beds) that are generally suitable for different types of mattresses and some examples of each of them in post #1 here.
Phoenix