Hi greeneggsandsam,
Most manufacturers don’t list the foam densities on their site but they will provide them when you talk with them and ask them.
No … any “red flags” would be based on the quality of the materials inside the mattress and on how they compare to your other finalists based on the parts of your personal value equation that are most important to you.
Those are the basics that I would pay most attention to and there is also more about the most important parts of the “value” of a mattress purchase in post #13 here and the posts it links to.
The foam density guidelines that I would suggest using are in post #4 here. In lower budget ranges where all the materials can’t be the “best” quality available then 1.5 lb polyfoam would certainly make a better choice than lowering the quality/density of the comfort layers which are usually the “weak link” of a mattress. If you are in a higher weight range that is more than about the lower 200 lb range or so and the mattress has thinner comfort layers so you would compress the support layers more then the density of the support layers would play a larger role in the durability of the mattress and using higher density materials would become more important.
Phoenix