Hi Mangosplums,
There are no “standard” definitions or consensus of opinions for firmness ratings in the industry 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. This is all relative and very subjective and is as much an art as a science.
I don’t think there is anyone that can answer this because most people in the industry would only be familiar with a relatively small number of memory foam mattresses out of the thousands of memory foam mattresses in the industry.
When you can test different mattresses in person in a store then you would be able to tell which of the mattresses you tested feels the softest to you based on your own personal experience (regardless of whether someone else would agree with your firmness ratings) but when you can’t test a mattress in person then the most reliable source of guidance is always a more detailed phone conversation with a knowledgeable and experienced retailer or manufacturer that has your best interests at heart and who can help “talk you through” the specifics of their mattresses and the properties and “feel” of the materials they are using (fast or slow response, resilience, firmness etc) and the options they have available that may be the best “match” for you based on the information you provide them, any local testing you have done or mattresses you have slept on and liked or other mattresses you are considering that they are familiar with, and the “averages” of other customers that are similar to you. They will know more about “matching” their specific mattress designs and firmness levels to different body types, sleeping positions, and preferences (or to other mattresses that they are familiar with) than anyone else.
In other words you will get some very conflicting opinions from different people with these types of very subjective questions because each person may have different answers based on their own personal experience and on the limited number of mattresses that they have personally tested and compared.
Phoenix