Hi Black Mammoth,
The first place I would start your research is the mattress shopping tutorial here which has all the basic information, steps, and guidelines that can help you make the best possible choice … and know how and why to avoid the worst ones.
The Bed in a Box Purity uses high quality materials (100% natural continuous pour Dunlop made by Mountaintop foam) so unlike their memory foam mattresses there aren’t any weak links in the design however for most people they probably wouldn’t be in the best value range compared to other similar mattresses that are available to the members here. While they do have a good return policy (although you need to wait for 60 days before returning a mattress which is a little bit on the long side) … they don’t appear to have a layer exchange so there isn’t a way to fine tune the mattress if all you need is to change the firmness of the comfort layer. They also don’t have any options to customize the firmness of the support core and their default firmness may be a little bit on the soft side for those that are in higher weight ranges. While they are certainly a reasonable option compared to many mainstream mattresses … for most people there would be better options available.
The tutorial post includes a link to a list of the members here that sell mattresses online (in the optional online step) and many of them (including Dreamfoam Bedding that you mentioned) make latex mattresses that use different types and blends of latex with a wide range of different designs, features, options, return/exchange policies, and price ranges which would also be well worth considering. Just as one example … Spindle Mattress here uses the same type of latex that BIAB uses (100% natural continuous pour Dunlop made by Mountaintop foam) except they use three 3" layers and all of the layers can be individually customized for firmness (rather than just the top layer) and they provide the option for a layer exchange if you need it as well. There are many other good options available on the list but I wanted to mention Spindle because they use exactly the same type of latex so they are a more “apples to apples” comparison with BIAB.
Latex is the most durable foam material in the industry and will soften less than other types of foam materials so I certainly wouldn’t be concerned about foam softening with any type of latex mattress. When you see reports of “foam softening” with latex … outside of the very rare material defects that can happen from time to time … it’s usually the result of someone who chooses a mattress that was “on the edge” of being too soft for their body type and sleeping positions so that even a very small amount of foam softening that will happen with any material (less with latex than other types of foam) was enough to put them “over the edge” of the comfort and support range that was suitable for them. These types of comments are almost always the result of making the wrong firmness choice in the first place rather than from the latex softening.
If you do decide to purchase this mattress then I would make sure that you purchase it from their Dreamfoam site rather than Amazon so that you have the benefit of their 10% discount (see their facebook page here) and their 45 day comfort guarantee as well.
You are also right that there can be some very big differences between mattresses that are in the same general category and there are even bigger differences between mattresses that are in different categories completely and there is more about latex vs memory foam mattresses in post #2 here.
When you can’t test a mattress in person the one of the most important parts of an online mattress purchase is a more detailed conversation with the retailer/manufacturer so they can help “talk you through” the options they have that would have the best chance of success based on the information you provide them, the mattresses you have tested, and the “averages” of other customers that are similar to you (see mattress firmness/comfort levels in post #2 here).
There is also more about the different ways to choose a mattress (online or local) and how to minimize the risks involved with each of them in post #2 here.
Phoenix