Hi Mr Ambien,
As you mentioned … Mattress 1st is a Serta house brand carried by a group of independent retailers who are part of a purchasing group. The value of any particular model such as the Royalville plush would depend on knowing the specifics of the layering (which being made by Serta is not likely to include the density of any polyfoam in the mattress which means a blind purchase with no way to know the durability of what you are buying). I would put them in the same group of “major brands” that I would tend to avoid because of the likelihood of lower density foams and lack of transparency of the ingredients in them.
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A good set of guidelines when purchasing a mattress includes avoiding the major brands or for that matter any brand or outlet that can’t or won’t show you the specifics of the materials in a mattress that you are considering. One of the “tests” of a better outlet is one who is happy to tell you the specific details of what is in their mattresses and actually wants you to know (so you can make meaningful comparisons) rather than makes it difficult or impossible to find out.
With a damaged shoulder and scoliosis, it becomes especially important to find the best balance between pressure relief (especially in your shoulders) and support/alignment (along the length of your spine) in all your sleeping positions that works best for you. It also means that the help and guidance of someone who will do more to educate you about the real differences between mattresses and how to find a mattress that “fits” your needs and preferences regardless of brand more they focus on trying to sell you something that they carry (and that makes them the most possible profit) can be your best friend. The best of these are usually at local factory direct outlets and local sleep shops that carry smaller local or alternative brands (not major brands that are made locally).
Some of the better choices in the Cincinnati area are in post #212 here.
Phoenix